The cover image was created by the transcriber and is placed in the public domain.

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Part of a transverse section of a Permian Osmundaceous Fern stem, Thamnopteris Schlechtendalii (Eichwald). a, outer xylem; b, inner xylem. For description, see page 329. (After Kidston and Gwynne-Vaughan. Very slightly reduced.)

I REGRET that pressure of other work has prevented the completion of this Volume within a reasonable time since the publication of Volume I. Had Volume II been written ten years ago, the discoveries made in the course of the last decade would have given an out-of-date character to much of the subject-matter. It is more especially in regard to the Ferns and the extinct members of the Gymnosperms that our outlook has been materially altered by recent contributions to Palaeobotany. It is, however, some satisfaction to be able to add that recent progress has been relatively slight in that part of the subject dealt with in the first volume.

The original intention was to complete the whole work in two volumes. Soon after the second volume was begun, it became evident that the remaining divisions of the plant-kingdom could not be included within the compass of a single volume. I decided, therefore, to take the consequences of having embarked on too ambitious a plan of treatment, and to preserve uniformity of proportion by reserving the seed-bearing plants for a third volume. The third volume will include the Pteridosperms, other than those briefly described in the final chapter of the present volume, and other classes of Gymnosperms. I propose also to devote such space as is available within the limits of a text-book to the neglected subject of the geographical distribution of plants at differentvi stages in the history of the earth. It is my intention to complete Volume III with as little delay as possible. As I have written elsewhere, the past history of the Flowering plants needs special treatment, and anything more than a mere compilation can be adequately attempted only after considerable research and with the assistance of botanists possessing a special knowledge of different families of Angiosperms. The need of a critical examination of available data in regard to the geological history of this dominant group will not be lost sight of.

I am well aware that while certain genera have received an undue share of attention in the present volume, others have been ignored or treated with scant consideration. For this inconsistency I have no excuse to offer, beyond the statement that the subject is a large one, and selection is necessary even though the work consists of three volumes.

The publication in 1909 of a collection of excellent photographs of Palaeozoic Plants, with brief descriptive notes, by Mr Newell Arber, as one of a series of popular “Nature Books,” bears striking testimony to the remarkable spread of interest in the study of the vegetation of the past, which is one of the outstanding features in the recent history of botanical science.

In the list of illustrations I have mentioned the source of all figures which have been previously published. I would, however, supplement the statement of fact with an expression of thanks to corporate bodies and to individuals who have allowed me to make use of blocks, drawings, or photographs.

I wish to thank my colleague, Mr A. G. Tansley, for placing at my disposal several blocks originally published in the pages of the New Phytologist. To Professor Bertrand of Lille and to his son Dr Paul Bertrand I am indebted for several prints andvii descriptive notes of specimens in their possession. My friends Dr Nathorst of Stockholm and Dr Zeiller of Paris have generously responded to my requests for information on various points. I wish especially to thank Dr Kidston for several excellent prints of specimens in his collection and for the loan of sections. I have profited by more than one examination of his splendid collection at Stirling. Professor Weiss has generously allowed me to borrow sections from the Manchester University collections, more especially several which have been reproduced in the chapter devoted to the genus Lepidodendron. To Professor F. W. Oliver my thanks are due for the loan of sections from the collection under his charge at University College. I have pleasure also in thanking Dr Scott, not only for lending me sections of a Lepidodendron and for allowing me to use some drawings of Miadesmia originally made by Mrs Scott for reproduction in his invaluable book, Studies in Fossil Botany, but for kindly undertaking the laborious task of reading the proofs of this volume. It would be unfair to express my gratitude to Dr Scott for many helpful suggestions and criticisms, without explicitly stating that thanks to a friend for reading proofs must not be interpreted as an attempt to claim his support for all statements or views expressed. The General Editor of the Series, Mr A. E. Shipley, has also kindly read the proofs. I am under obligations also for assistance of various kinds to Prof. Thomas of Auckland, New Zealand, to Mr Boodle of Kew, to Mr D. M. S. Watson of Manchester, to Mr T. G. Hill of University College, and to Mr Gordon of Emmanuel College, Cambridge. I am indebted to the kind offices of Miss M. C. Knowles for the photograph of the specimen of Archaeopteris hibernica in the Irish National Museum, Dublin, reproduced on page 561.

viii

Many of the illustrations are reproduced from drawings by my wife: those made from the actual specimens are distinguished by the addition of the initials M. S. I am grateful to her also for some improvements in the letter-press. For the drawings made from sections and for some of the outline sketches I am responsible. I have availed myself freely of the facilities afforded by Professor McKenny Hughes in the Sedgwick Museum of Geology for the examination of specimens under the charge of Mr Newell Arber, the University Demonstrator in Palaeobotany. It is a pleasure to add that, as on former occasions, I am indebted to the vigilance of the Readers of the University Press for the detection of several errors which escaped my notice in the revision of the proofs.

• Page   16, line 4. For “The North American Tulip tree” read The Tulip tree of North America and China.
•     „      66, line 2 from the bottom. For “Browera” read Berowra.
•     „    127, line 3 and 4 from bottom. For Achyla and Palaeachyla read Achlya and Palaeachlya.
•     „    145, lines 4 and 5. For “Upper Greensand” read Lower Eocene.
•     „    162, line 3 from bottom. For “Corallina barbata” read Cymopolia barbata.
•     „    170, line 20. For “sporangiaphore” read sporangiophore.
•     „    185, line 2. The genera Udotea and Halimeda, members of the Siphoneae, are incorrectly included under the Corallinaceae.
•     „    191, line 11 from bottom. Omit Chondrus crispus, which is one of the Florideae and not a Brown Alga.
•     „    202, line 13. For “Halmeda” read Halimeda.
•     „    250, line 11. For “three” read the.
•     „    381, line 10. For “Calamopytus” read Calamopitys.

Sphenophyllum.

The account of the Sphenophyllales given in the first volume[2] of this work must be extended and somewhat modified in the light of recent work on the fertile shoots of Sphenophyllum.

Sphenophyllostachys Dawsoni (Will.) was described as consisting of an axis bearing superposed whorls of bracts connate at the base in the form of a shallow funnel-shaped collar giving off from the upper surface and close to the axis of the cone two concentric series of sporangiophores. Occasionally there are three series, as represented in fig. 112. In another type of strobilus, Sphenophyllostachys Römeri[3] each sporangiophore terminates in two pendulous sporangia (fig. 113, A; see also fig. 107, C, vol. I.). It has already been pointed out that the common occurrence of detached strobili necessitates their description under distinct specific names; it is only by a rare accident that we can assign fossil cones to their vegetative shoots. There are, however, reasons for believing that Sphenophyllostachys Dawsoni is the strobilus of the plant originally described by Sternberg[4] from impressions of foliage-shoots as Rotularia cuneifolia. Another difficulty presented by petrified material is that of determining, with certainty, whether two imperfect specimens, differing from one another in features which do not appear to 2be of sufficient importance to warrant specific separation, are forms of one species or portions of specifically distinct cones. It has been pointed out by Scott[5] that the strobilus known as Sphenophyllostachys Dawsoni probably includes two distinct species, one being the cone of Sphenophyllum cuneifolium Sternb., and the other the cone of S. myriophyllum Crép[6]. The stem of S. myriophyllum agrees anatomically with the type known as Sphenophyllum plurifoliatum Will. and Scott[7].

Fig. 112. Sketch of a radial longitudinal section of Sphenophyllostachys. There are usually two concentric series of sporangia on the sporophylls, not three as shown in the figure. The upper figure (after Zeiller) shows the linear bracts in surface-view.

In addition to the two types of cone already mentioned, Sphenophyllostachys Dawsoni and S. Römeri, others have been described by Kidston from carbonised impressions. One of these is the fertile branch of Sphenophyllum majus[8]. The basal portions of the bracts of each whorl form a narrow collar round the axis of the cone; the free portion of each bract consists of a lamina divided into two equal bifid lobes bearing 3on its upper surface one group, or possibly two groups, of four sessile sporangia between the narrow coherent bases of the laminae and the sinus between the terminal lobes (fig. 113, C). Another characteristic feature is the greater length of the internodes; this renders the cone less compact and less sharply differentiated from the vegetative shoots than those of other species. A specimen in Dr Kidston’s collection illustrates the peculiar character of the fertile portion of this species; it consists of an axis bearing a succession of lax sporophylls succeeded above and below by whorls of sterile leaves. In this species, therefore, we cannot speak of a compact strobilus at the end of a shoot of limited growth, but of axes in which sterile and fertile leaves are borne alternately[9], a condition recalling the 4alternation of foliage leaves and sporophylls in Tmesipteris and in Lycopodium Selago.

Fig. 113.

1. Sphenophyllostachys Römeri. (Solms-Laubach.)
2. Sphenophyllum trichomatosum Stur.
3. Sphenophyllum majus. Bronn. (A–C. After Kidston.)

Another form of cone, also from the Middle Coal Measures, is referred by Kidston to Sphenophyllum trichomatosum Stur[10] (fig. 113, B): this is characterised by the more horizontal position of the bracts, which “do not appear to be so much or so suddenly bent upwards in their distal portion as in some other species of Sphenophyllum,” and by sessile sporangia borne singly on the upper face of each bract.

Fig. 114. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a node in longitudinal section, showing one sporophyll and the base of the opposite one. v.l. ventral lobe of sporophyll; v.s. one of the segments into which it divides; v.s′. stump of another segment; d.l. dorsal lobe; d.s., d.s′. segments of dorsal lobe.

A more recent addition to our knowledge of the fertile shoots of Sphenophyllum is due to Scott who has described a new type of cone under the name Sphenophyllum fertile[11]. The petrified specimen on which the species was founded was discovered by Mr James Lomax in the Lower Coal Measures of Lancashire; it represents a portion of a cone 6 cm. long and approximately 12 mm. broad. The axis contains a single vascular cylinder agreeing in essentials with the type of stem structure known as Sphenophyllum plurifoliatum. The nodal regions, which exhibit the slight swelling characteristic of the genus, bear several (probably twelve) appendages connate at 5the base and forming a narrow flange encircling the axis. Each bract, the base of which forms part of the narrow collar surrounding the axis, consists of two lobes, ventral and dorsal, divided palmately into several (sometimes four) segments or sporangiophores (fig. 115). Each sporangiophore terminates distally in an oblong or oval lamina bearing two sporangia on its adaxial face (fig. 114). The space between the axis and the periphery of the cone is thus occupied by crowded peltate laminae, each with its pair of sporangia. A single vascular bundle supplies each sporangiophore and bifurcates in the distal lamina into two branches which extend to the bases of the sporangia. The sporangia agree in structure with those of other species of Sphenophyllum: the spores are of one size and elliptical, characterised by the presence of several sharp ridges or flanges encircling the spore-wall in the direction of the major-axis. Sphenophyllostachys fertilis differs from all previously recorded types in the absence of sterile bracts. The appendages of the cone-axis are all fertile, a striking contrast to the differentiation into protective and sporangia-bearing bracts which constitutes a constant feature in the cones of Sphenophyllum and Calamites. It is possible, as Scott suggests, that the absence of sterile segments is the result of modification of the6 more usual type of strobilus; instead of the dorsal and ventral lobes of the bracts sharing between them the duties of protection and spore-production, the whole of each bract is constructed on the plan of the maximum spore-output, the laminar terminations of the sporangiophores serving the purpose of protection. The cone may be described as more specialised than the normal type of strobilus for reproductive purposes[12].

Fig. 115. Sphenophyllostachys fertilis (Scott). (After Scott.) Diagram of a single sporophyll as it would appear in a transverse section of the cone; showing one lobe (dorsal or ventral). ax, part of axis to which the sporophylls are attached.

Fig. 116. Sphenophyllostachys Dawsoni. (After Thoday.) A. Larger spores; B, abortive spores; C, mature spores showing the characteristic spines.

It has been stated, on evidence which is unsatisfactory, that Sphenophyllum possesses two kinds of spores. While regarding the genus as homosporous on the evidence before us, it is interesting to find that cases occur in which the spores in the same sporangium exhibit a marked difference in size. Attention has been called by Williamson and Scott[13] to variation in the dimensions of spores: a more pronounced difference in size has been recorded by Mr Thoday[14] who gives 120μ as the maximum and 90μ as the minimum diameter of the spores in a cone of Sphenophyllostachys Dawsoni. The presence of several abortive spores in the sporangium (fig. 116) containing the larger spores favours the view that this difference in size may be the first step towards the development of heterospory.

7

It is clear that the types of strobilus designated Sphenophyllostachys (figs. 112–114) present a divergence of characters too great to be comprised under one genus; but in the absence of fuller information, we cannot do otherwise than follow the only logical custom of grouping them together as examples of strobili borne by plants which, in the present state of our knowledge, are most conveniently referred to the genus Sphenophyllum.

Cheirostrobus.

This generic name was applied by Dr Scott[15] to a calcified cone obtained by Mr James Bennie in 1883 from the Lower Carboniferous plant-beds of Pettycur near Burntisland on the Firth of Forth. Cheirostrobus is distinguished from Sphenophyllostachys by its greater breadth (3.5 cm.); externally it agrees more closely with the fertile shoots of Lepidodendron than with those of Sphenophyllum. A single vascular cylinder having the form of a fluted Doric column (fig. 117, B, x) occupies the axis of the cone: it consists for the most part of reticulate tracheae which tend to assume a short or isodiametric form in the central region; the smaller protoxylem tracheids with the spiral form of pitting constitute the sharp and prominent ridges at the periphery of the xylem-cylinder. In the outer part of the cylinder the metaxylem[16] consists exclusively of tracheae, but towards the centre of the axis these are associated with numerous parenchymatous cells.

The xylem is therefore centripetal in origin as in Sphenophyllum and in nearly all recent and fossil members of the Lycopodiales. In the type-specimen of Cheirostrobus the vascular cylinder of the cone consists entirely of primary xylem, but secondary xylem has been found in a more recently discovered specimen[17]. Secondary xylem occurs also in the peduncle of the cone. No appreciable remains of phloem have 8been found. The cortex consists of slightly elongated rather thick-walled tissue containing secretory sacs. Crowded superposed whorls of bracts (or sporophylls), usually twelve in each whorl, are borne on the axis and each sporophyll receives a single vascular bundle from one of the vertical ridges of the xylem column (fig. 117, A, lt). The members of each whorl are connate at the base: from this narrow collar each sporophyll branches into an upper or dorsal and a lower or ventral limb (fig. 117, A, f and s). Each limb divides palmately at a short distance from its origin into three slender segments, which extend in a horizontal direction and terminate in large laminar expansions (fig. 117, B, s) to afford a protective covering to the surface of the cone. The upper set of three segments, constituting9 sporangiophores (fig. 117, A, B, f) or fertile divisions of the sporophyll, expand distally into comparatively bulky laminae; each of these bears on its adaxial face four diagonally placed outgrowths which form the short pedicels of very long and narrow sporangia. The three lower segments—the sterile divisions of the sporophylls—(fig. 117, A, B, s) are similar to the upper set except in their greater length and in the kite-shaped form of their distal laminae which are provided with lateral lobes. The single vascular strand which supplies each sporophyll is represented at lt in fig. 117, B; at lt′ the strand has divided into four, the three upper bundles in the figure supply the sterile segments and the single lower bundle ultimately divides into three which supply the fertile segments. A pair of blunt processes (fig. A, s) extend downwards over the ends of the underlying fertile lamina and two slender prolongations extend upwards through several internodes.

Fig. 117. A, B. Cheirostrobus pettycurensis Scott. (After Scott.)
C, D. Pseudobornia ursina Nath. (After Nathorst.)

1. Diagrammatic radial longitudinal section of part of the cone-axis and two sporophylls. lt, bundle passing out to sporophyll; f, fertile segment of sporophyll showing two sporangia; s, sterile (lower) segment.
2. Part of transverse section. x, stele; lt, lt′, bundles on their way to sporophylls; a, tips of sterile segments of lower sporophylls.
3. Palmately branched leaf (½ natural size).
4. Node of stem showing leaf-bases.

An economical arrangement of the long and narrow sporangia and of the sporophyll-segments between the axis and the periphery of the cone is rendered possible by the interlocking of the sterile and fertile segments by means of a groove in the upper face of the latter for the accommodation of the former. The sporangia are characterised by their unusually long and narrow form: the length of a sporangium may reach 1 centimetre. In the structure of the wall the sporangia of Cheirostrobus agree closely with those of Calamostachys[18] and Sphenophyllostachys. The spores are of one size only. The vascular cylinder of the peduncle, originally described by Williamson[19] as the peduncle of a large Lepidostrobus (the cone of Lepidodendron), is characterised by the presence of a short radially disposed zone of secondary tracheids, a feature, as Scott points out, which may extend into the axis of the cone. It is noteworthy that the protoxylem elements are not always external, but occasionally occur internal to one or two of the outermost metaxylem tracheae: the usual exarch[20] structure of 10the central cylinder is not therefore absolutely constant, but may be replaced by a mesarch arrangement.

The presence of a few sterile leaves on the peduncle below the fertile portion of the cone, which agree in their lobed laminae with the sporophylls, is the only fact which we possess as to the form of the vegetative characters of the genus.

The above description is sufficient to indicate the extraordinary complexity and high degree of specialisation of Cheirostrobus. The sporophylls, with their trilobed segments, and the crowded sporangia of exceptional length attached only by a narrow base constitute striking peculiarities of the genus.

It is unfortunate that we are still without any satisfactory evidence as to the nature of the plant the cones of which have been made the type of a new genus and a new family. Cheirostrobus affords an interesting example of a type of reproductive shoot constructed on a plan sui generis, and may be classed with some other extinct genera as instances of the production in the course of evolution of architectural schemes which appear to have been ill adapted for competition with equally efficient though much simpler types. But the discovery of these isolated forms of restricted geological range among the relics of the Palaeozoic vegetation frequently supplies a key to phylogenetic problems. Cheirostrobus by its complex combination of features characteristic of the Equisetales, the Lycopodiales and the genus Sphenophyllum throws a welcome light on the inter-relationships of groups which represent divergent series. The combination of morphological features in this generalised type led the author of the genus to describe it as a descendant of an old stock which existed prior to the divergence of the Equisetales and Lycopodiales.

The discovery of this new type of strobilus naturally led to a search among Lower Carboniferous plants for vegetative shoots exhibiting characters conformable with the whorled and branched leaves of Cheirostrobus. In Sphenophyllum we have a genus obviously comparable with Cheirostrobus as regards the form and disposition of the leaves, but the differences between the cones and the striking similarity of the vascular cylinder of the latter to that of Lepidodendron demonstrate conclusively that11 we must look elsewhere for the vegetative members of the plant which produced cones of the Cheirostrobus type.

In 1902 Professor Nathorst[21] instituted the generic name Pseudobornia for plants of which imperfect examples had previously been referred by Heer[22] to Calamites under the name C. radiatus. Heer’s plants were obtained from Upper Devonian rocks of Bear Island in the Arctic seas and additional specimens were brought from the same locality by the Swedish Polar Expedition of 1898. Pseudobornia possesses jointed stems (fig. 117, D) bearing whorled and shortly stalked leaves, often four in number, at each node. The leaves are palmately branched with fine serrated edges (fig. 117, C). Certain specimens, which are no doubt correctly described by Nathorst as cones, are characterised by a thick axis bearing whorled leaves with sporangia on their lower surfaces, but the material is not sufficiently well preserved to render possible a recognition of structural details. It has been suggested by Scott that Pseudobornia may possibly be referable to the Sphenophyllales and that the stem of Cheirostrobus “may have had something in common with” Nathorst’s genus[23]. The beds in which the stems occur are of Upper Devonian age, while Cheirostrobus was found in Lower Carboniferous rocks: this difference in age is not, however, a serious objection to the validity of the comparison. We cannot do more than express the view that Pseudobornia, so far as can be ascertained without an examination of petrified material or of more perfect impressions of strobili, exhibits vegetative features not inconsistent with the morphological characters of the fertile shoots known as Cheirostrobus.

The institution of a special group-name for the reception of Sphenophyllum is justified by the sum of its morphological features, which do not sufficiently conform to those of any existing group of Pteridophytes to warrant its inclusion in a system of classification based on recent genera. In the case of Cheirostrobus we are limited to the characters of the cone and 12its peduncle. The suggestion that the Devonian fossils known as Pseudobornia may represent the foliage shoots of a plant closely related to Cheirostrobus has still to be proved correct. Although we may find justification in the highly complex and peculiar structure of Cheirostrobus for the recognition of the genus as a type of still another group of Pteridophytes, it would be unwise to take this step without additional knowledge.

The undoubted similarity between Cheirostrobus and Sphenophyllum coupled with striking points of difference favours the inclusion of the two genera in distinct families placed, for the present at least, in the group Sphenophyllales.

Group SPHENOPHYLLALES.

It has recently been proposed to include the family Psilotaceae, comprising the two recent genera Psilotum and Tmesipteris, as another subdivision of the Sphenophyllales. This proposal had been made by Professor Thomas[24] primarily on the ground that the sporophylls of Tmesipteris and Psilotum appear to afford the closest parallel among existing plants to the peculiar form of sporophyll characteristic of the Sphenophyllales. The morphological interpretation of the sporophylls of both Sphenophyllum and Cheirostrobus has been the source of considerable discussion[25]. If we regard each sporophyll as a leaf with two lobes, one fertile and one sterile, except in the case of Sphenophyllostachys fertilis in which both are fertile, an obvious comparison may be made with the fern Ophioglossum; but the difference between a single fern frond, consisting of a comparatively large sterile lamina bearing a fertile branch composed of a long axis with two rows of sporangia embedded in its tissues, and the whorled sporophylls of Sphenophyllum is considerable.

A brief reference may be made to the principal reasons which have led to the suggestion that the Psilotaceae should be included 13in the Sphenophyllales. The shoots of Tmesipteris bear simple foliage leaves spirally disposed on a slender axis, and in association with these occur sporophylls consisting of a short axis bearing a pair of small lobes and a bilocular synangium[26] (fig. 120, B). The synangium is seated on a very short stalk given off from its sporophyll at the base of the pair of laminae: the synangium with its short stalk may be spoken of as the sporangiophore. In most cases the synangium appears to be sessile on the sporophyll, but occasionally the much reduced stalk is prolonged and forms an obvious feature. Dr Scott[27] suggested that the Tmesipteris synangium with its axis may correspond to the ventral lobe (or sporangiophore) of Sphenophyllum. In the latter genus the whorled sporophylls consist in most species of a dorsal and a ventral lobe, the latter serving as a sporangiophore bearing one or more sporangia; in Tmesipteris the sporophylls are spirally disposed and each consists of a bilobed sterile portion bearing a septate sporangium or bilocular synangium on a very short ventral lobe. Professor Bower[28], in his account of the development and structure of the sporophylls of Tmesipteris, drew attention to the comparatively frequent occurrence of abnormal sporophylls and spoke of the plant as unstable. More recently Professor Thomas[29] of Auckland has carefully examined living plants, with the result that variations of different kinds are proved to be exceedingly common. He finds that sporophylls occur which exhibit repeated dichotomy of the axis (fig. 120, D, F) and thus each may bear four instead of two leaf-lobes and three synangia, one at the first fork and one at each of the forks of the second order[30].

Other abnormalities occur in which the synangium is raised on a distinct stalk instead of being more or less sessile at the point from which the leaf-lobes diverge. A third form of departure from the normal is that in which there is no synangium on the bilobed sporophyll, its place being taken by a leaf-lobe. The deduction from the occurrence of these abnormalities is that the synangium of Tmesipteris represents a ventral leaf-lobe,14 as Scott suggested. Professor Thomas draws attention to the resemblance between Tmesipteris sporophylls and the foliage-leaves of Sphenophyllum, which are either simple with dichotomously branched veins or the lamina is deeply divided into two or more segments. In some types of Sphenophyllostachys the bracts are simple (S. Dawsoni), but in others (Sphenophyllum majus, fig. 113, C) they are forked like the foliage-leaves and bear a close resemblance to the abnormal sporophylls of Tmesipteris. Moreover, in Sphenophyllostachys Römeri (fig. 113, A) each ventral lobe of a sporophyll bears two sporangia, a condition almost identical with that represented by the occasional occurrence of a synangium on a comparatively long stalk in Tmesipteris. Similarly the more elaborate sporophylls of Cheirostrobus may be compared with the branched sporophylls of Tmesipteris (fig. 120). This agreement between the sporophylls of the Palaeozoic and recent genera acquires additional importance from the very close resemblance between the exarch stele of Sphenophyllum and that of the genus Psilotum, which conforms to the Palaeozoic type not only in the centripetal character of the primary xylem and in its exarch structure, but also in the occasional occurrence of secondary xylem[31], and in the stellate form of its transverse section. The occasional mesarch structure of the stele of Cheirostrobus finds a parallel in the mesarch xylem groups in the stem of Tmesipteris. It is thus on the strength of these resemblances that Thomas and Bower would remove the Psilotaceae from the group Lycopodiales and unite them with Sphenophyllum and Cheirostrobus in the Sphenophyllales. While admitting the validity of the comparison briefly referred to above, I prefer to retain the Psilotaceae as a division of the Pteridophyta including only Psilotum and Tmesipteris.

In his recent book on The Origin of Land Flora, Prof. Bower raises objection to the use of the term ventral lobe in speaking of the sporangium-bearing stalk or sporangiophore borne on the sporophyll of Sphenophyllum. He points out that the use of this term implies the derivation of the sporangiophore by metamorphosis of part of a vegetative leaf, an opinion untenable 15in the absence of proof. The designation sporangiophore is no doubt preferable to that of ventral lobe as it carries with it no admission of particular morphological value; as a further concession to a non-committal attitude we may provisionally at least regard a sporangiophore as an organ sui generis “and not the result of modification of any other part[32].”

The view put forward by Prof. Lignier[33] that the Sphenophyllales are descendants of primitive ferns is not convincing, and his comparison of Sphenophyllum with Archaeopteris lacks force in view of our ignorance as to the nature of the reproductive organs of the latter genus. That the Sphenophyllales are connected with the Equisetales and with the Psilotales by important morphological features is clear; but the comparison between the sporophylls of the extinct genera with those of the existing genus Tmesipteris, though helpful and possibly based on true homology, cannot be considered as settling the morphological value of the sporangiophores of Sphenophyllum and Cheirostrobus.

I do not propose to discuss at length the different views in regard to the morphological nature of the sporangiophore of Sphenophyllum. The comparison, which we owe in the first instance to Scott, with the synangium of the Psilotales with its short stalk, though not accepted by Lignier as a comparison based on true homology, is one which appeals to many botanists and is probably the best so far suggested. The further question, whether these sporangiophores are to be called foliar or axial structures is one which has been answered by several authors, but it is improbable that we shall soon arrive at a decision likely to be accepted as final. Discussions of this kind tend to assume an exaggerated importance and frequently carry with them the implication that every appendage of the nature of a sporangiophore can be labelled either shoot or leaf. We treat the question from an academic standpoint and run a risk of ignoring the fact that the conception of stem and leaf is based on morphological characteristics, which have been evolved as the result of gradual differentiation of parts of one originally 16homogeneous whole. There is much that is attractive in the view recently propounded by Mr Tansley that a leaf is not an appendicular organ differing ab initio from the axis on which it is borne, but that it is in phylogenetic origin a “branch-system of a primitive undifferentiated sporangium-bearing thallus[34].” Admitting the probability that this view is correct, our faith in the importance of discussions on the morphological nature of sporangiophores is shaken, and we realise the possibility that our zeal for formality and classification may lead to results inconsistent with an evolutionary standpoint[35].

PSILOTALES.

The two recent genera Psilotum and Tmesipteris are usually spoken of as members of the family Psilotaceae which is included as one of the subdivisions of the Lycopodiales. It is probable, as Scott[36] first suggested, that these two plants are more nearly allied than are any other existing types to the Palaeozoic genus Sphenophyllum.

We may give expression to the undoubted resemblances between Tmesipteris and Psilotum and the Sphenophyllales by including the recent genera as members of that group, originally founded on the extinct genus Sphenophyllum; this is the course adopted by Thomas[37] and by Bower[38]: or we may emphasise the fact that these two recent genera differ in certain important respects from Lycopodium and Selaginella by removing them to a separate group, the Psilotales. The latter course is preferred on the ground that the inclusion of Psilotum and Tmesipteris in a group founded on an extinct and necessarily imperfectly known type, is based on insufficient evidence and carries with it an assumption of closer relationship than has been satisfactorily established.

The genus Tmesipteris (fig. 120, A) is represented by a single species T. tannensis Bertr.[39] which usually occurs as an epiphyte on the stems of tree-ferns in Australia, New Zealand, and Polynesia. Psilotum, with two species P. triquetrum 18Sw. (fig. 118) and P. complanatum Sw., flourishes in moist tropical regions of both hemispheres, growing either on soil rich in organic substances or as an epiphyte. Both genera are considered to be more or less saprophytic.

Fig. 118. Psilotum triquetrum (½ natural size).

1. Synangium.
2. Sporophyll after removal of the synangium. (M.S.)

Psilotum. The common tropical species P. triquetrum (fig. 118) is characterised by an underground rhizome which forms a confused mass of dark brown branches covered with filamentous hairs as substitutes for roots and gives off erect repeatedly forked aerial shoots. In P. complanatum[40] the habit 19is similar to that of the more abundant and better-known species, but the pendulous shoots are characterised by their broader and flatter form. In both species the function of carbon-assimilation is performed by the outer cortex of the green branches, as the small size of the widely-separated foliage leaves renders them practically useless as assimilating organs.

The sporophylls consist of a short axis terminating in two small divergent forks and bearing on its adaxial surface a trilocular or in rare cases a bilocular synangium (fig. 118, A and B). The walls of the loculi are composed of several layers of cells and dehiscence takes place along three lines radiating from the centre of the synangium. Professor Thomas[41] has recorded “fairly numerous instances in Psilotum of a second dichotomy of one branch of the first fork, or, less frequently, of both branches”: instead of one synangium subtended by the two slender leaflets of the forked sporophyll-axis, there may be two synangia and three leaf-lobes or three synangia and four leaf-lobes. The occurrence of both these abnormalities in Psilotum and Tmesipteris shows a decided tendency in the Psilotales to a repeated dichotomy of the sporophylls[42].

A single stele[43] with a fluted surface occupies the axis of an aerial shoot (fig. 119, A); the axial region is occupied by a core of elongated mechanical elements (s), which may occasionally extend to the periphery of the xylem and break the continuity of the band of scalariform tracheae (fig. 119, A, a). The tracheae form the arms of an irregularly stellate stele and each arm is terminated by protoxylem elements (fig. 119, B, px). The rays of the xylem cylinder, which may be as many as six or eight in the upper part of the aerial shoots, become reduced in number as the rhizome is approached, assuming a diarch structure near the junction. In the rhizome the xylem forms an approximately triangular group of tracheae without any core of mechanical elements. Three to four layers of parenchyma20 succeeded externally by an ill-defined phloem (fig. 119, A, p) surround the xylem and a fairly distinct endodermis (fig. 119, A and B, e) encloses the whole. To Mr Boodle[44] is due the 21interesting discovery that in some parts of the rhizome the parenchymatous zone surrounding the scalariform tracheae may become the seat of meristematic activity which results in the production of secondary tracheae often characterised by a sinuous longitudinal course. There is no definite cambium, but the radially disposed tracheae and the adjacent parenchymatous elements clearly demonstrate the secondary nature of the tissue immediately external to the group of primary xylem. Fig. 119, C, drawn from a section kindly supplied by Mr Boodle, shows the secondary xylem elements at x² associated with radially disposed thin-walled cells abutting on the primary xylem, x¹. It is probable that this added tissue may be a remnant of a more extensive secondary thickening characteristic of the ancestors of the recent species. In their manner of occurrence and sinuous course these secondary tracheids bear a resemblance to the secondary xylem of Lepidodendron fuliginosum[45]. The stele of the aerial shoot bears a fairly close resemblance to the vascular axis of Cheirostrobus, and its three-rayed form in the lower portions of the green branches recalls that of the Sphenophyllum stele, except that the axial xylem elements of the Palaeozoic genus are usually represented in Psilotum by mechanical tissue. The cortex consists of three regions (fig. 119, A), an outer zone of chlorophyllous tissue (a) rich in intercellular spaces succeeded by a band of mechanical tissue (b) which gradually passes into an inner region of larger and thinner-walled cells (c).

Fig. 119.

1. Diagram of transverse section of aerial shoot of Psilotum triquetrum. a—c, cortex; p, phloem; e, endodermis; s, stereome; x, xylem; a, gap in xylem.
2. Enlarged view of one of the angles of the xylem shown in A. px, protoxylem.
3. Part of transverse section of an approximately triangular rhizome stele showing a portion of the metaxylem x¹; px, protoxylem elements; x², secondary xylem.

The genus Tmesipteris[46] agrees with Psilotum in general habit and in its epiphytic and probably in some degree saprophytic mode of life. Its brown rootless rhizome, which grows among the roots of tree-ferns or rarely in the ground, gives off pendulous or erect shoots reaching a length of two feet and bearing lanceolate mucronate leaves 2–3 cm. long (fig. 120, A) attached by decurrent leaf-bases. The sporophylls, replacing the upper leaves or occurring in more or less well-defined zones alternating with the foliage leaves, consist of a short axis terminating in a pair of lanceolate lobes and bearing on its 22adaxial surface an elongated bilocular synangium attached to a very short stalk (fig. 120, B). Reference has already been made to the divergent opinions as to the morphological nature of the sporophylls or sporangiophores, but recent investigations23 distinctly favour the view that a sporophyll is best interpreted as a stalked leaf with two sterile laminae and an almost sessile, or in some cases a more obviously stalked, synangium; the whole sporophyll is characterised by the possession of a ventral and a dorsal lobe[47]. The drawings reproduced in fig. 120, D and F, illustrate some of the frequent variations described by Thomas in plants which he observed in the New Zealand forests. The sporophyll shown in fig. 120, D and F, has branched twice and bears three synangia.

Fig. 120. Tmesipteris.

• A. Foliage leaves.
• B. Sporophyll and bilocular synangium.
• C. Diagram of transverse section of stele. px, protoxylem.
• D, F. Abnormal sporophylls. (From drawings made by Prof. Thomas and generously placed at my disposal. A.C.S.)
• E. Portion of C enlarged.

The aerial branches of Tmesipteris possess a central cylinder of separate xylem groups in which the protoxylem occupies an internal position (fig. 120, C and E, px) enclosing an axial parenchymatous region. The cells of a few layers of the inner cortex immediately outside the endodermis are rendered conspicuous by a dark brown deposit. The cortex as a whole is composed of uniform parenchymatous tissue. In the lower part of the aerial shoots and in the rhizome the xylem forms a solid strand without protoxylem elements and conforms more clearly to that of Psilotum.

In this short account of the anatomy of Tmesipteris no mention is made of the effect produced on the stele by the departure of leaf-traces and of vascular stands to supply branches. Miss Sykes[48] in a recently published paper on the genus has shown that the exit of a leaf-trace does not break the continuity of the xylem of the stele, while the exit of a sporophyll-trace is marked by an obvious gap. Evidence is adduced in support of the conclusion that this difference, which at first sight appears to be one of morphological importance, is in reality merely a question of degree and “is due to the earlier preparation for the formation of ‘sporophyll’ than leaf-traces.” Miss Sykes gives her adherence to the view that the “sporophylls” of Tmesipteris are branches and not leaves, but despite the arguments advanced this interpretation seems to me less probable than that which recognises the sporophyll as a foliar organ. Prof. Lignier[49] has pointed out that if Miss Sykes’s conclusion as to the axial nature of the sporophyll in Tmesipteris is accepted, it diminishes the force of the comparison24 between the sporophylls of that genus and Sphenophyllum as those of the latter can hardly be regarded as other than foliar organs.

Both members of the Psilotales may, as Boodle has suggested, be regarded as descendants of a common parent in which the aerial stems possessed a fluted or stellate cylinder of mesarch xylem. There can be no doubt as to the significance of the morphological resemblances between the Psilotales and the genera Sphenophyllum and Cheirostrobus, but the position of Tmesipteris and Psilotum in the plant-kingdom may probably be best expressed by adopting the group-name Psilotales rather than by transferring the recent genera to the Sphenophyllales. One of the most striking differences between the Psilotales and the genus Lycopodium is in the form of the sporophylls and sporangia; in Lycopodium a single sporophyll bears a unilocular sporangium, but in the Psilotales the sporophyll may be described as a bilobed structure homologous with a foliage-leaf, bearing a sporangiophore which consists of a short stalk terminating in a bilocular or trilocular synangium; the short stalk receives a special branch from the vascular bundle of the sterile portion of the sporophyll[50].

Fossils described by authors as being closely allied to Psilotum.

A search through palaeobotanical literature reveals the existence of a very small number of specimens which have been identified as representatives of the Psilotales. An inspection of the material or published drawings leads one to the conclusion that practically no information of a satisfactory kind is available in regard to the past history of the two southern genera Psilotum and Tmesipteris, which are regarded by some botanists as relics of an ancient branch[51] of pteridophytes.

In 1842 Münster[52] instituted the genus Psilotites for a small impression of a slender branched axis from Jurassic rocks near Mannheim in Germany which he named Psilotites filiformis; 25Schimper[53] spoke of the specimens as too doubtful for determination, an opinion with which every botanist would cordially agree. Goldenberg’s species Psilotites lithanthracis[54] from the Saarbrücken coal-field is founded on impressions of axes: some of these are dichotomously branched and bear small oval projections, which may be rudimentary leaves or possibly leaf-scars. More recently Kidston[55] described specimens of branched axes from the Lanarkshire coal-field bearing a row of lateral thorn-like projections under the title Psilotites unilateralis; but these fragments, as Dr Kidston himself admits, are of no botanical value.

In a paper on fossil Salvinias, Hollick[56] mentions Salvinia reticulata, originally described by Heer and by Ettingshausen and S. Alleni Lesq.[57] a Tertiary species, and calls attention to their very close resemblance in form, nervation, and apex to the leaves of the genus Tmesipteris: he refers both species to that genus. The drawings reproduced by Hollick represent leaves with a midrib and numerous anastomosing lateral veins, whereas in Tmesipteris the lamina of the leaf has a midrib without lateral branches. An enlarged drawing of the outlines of the epidermal cells would correspond closely with the small reticulations in the fossil leaves and it may be that there has been some confusion between veins and cell-outlines. In any case there would seem to be no reason for the use of the recent generic name[58].

Among other fossils assigned to the Psilotales we have Marion’s genus Gomphostrobus from the Permian of France and Germany[59]. Marion placed this plant in the Coniferales on the strength of its resemblance to Walchia and Araucaria, but Potonié[60] is inclined to recognise in the leaves and monospermic sporophylls characters suggestive of Lycopodiaceous affinity.

26

The latter author in 1891[61], in ignorance of Marion’s proposal to adopt the name Gomphostrobus, instituted a genus Psilotiphyllum for the sporophylls of a species originally described by Geinitz[62] as Sigillariostrobus bifidus, but he subsequently adopted Marion’s designation and with some hesitation included the French and German specimens in the Psilotales. As stated elsewhere[63], Potonié’s arguments in favour of his view hardly carry conviction, and it is probably more in accordance with truth to deal with Gomphostrobus in the chapter devoted to the Coniferales.

Psilophyton.

The generic title Psilophyton, instituted by the late Sir William Dawson[64], has become familiar to geologists as that of a Pre-Carboniferous plant characteristic of Devonian and Silurian rocks in Canada, the United States of America, and Europe. From the botanist’s point of view the name stands for miscellaneous remains of plants of different types and in many cases unworthy of record. The genus was founded on impressions of branched axes from the Devonian strata of New Brunswick resembling the rachis and portions of lateral pinnae of ferns or the forked slender twigs of a Lycopod. The type-species Psilophyton princeps Daws. as represented on somewhat slender evidence in Dawson’s restoration, which accompanies the original description of the genus and has since been copied by several authors, is characterised by the possession of a horizontal rhizome bearing numerous rootlets and giving off dichotomously branched aerial shoots with spinous appendages, compared with rudimentary leaves, and terminating in slender branchlets bearing pendulous oval “spore-cases” from their tips. Some of the branchlets exhibit a fern-like vernation. The plant is spoken of by Dawson as apparently a generalised type[65], resembling in habit and in its rudimentary leaves the recent genus Psilotum and presenting points of contact with ferns. 27Specimens were found in an imperfectly petrified state showing a central cylinder of scalariform tracheae surrounded by a broad cortical zone of parenchyma and fibrous tissue.

Among other species described by the author of the genus we need only mention Psilophyton robustius, characterised by vegetative shoots and “spore-cases” similar to those of the type-species; but, as Solms-Laubach[66] has pointed out, the petrified sections referred by Dawson to P. robustius are of an entirely different anatomical type from that of P. princeps[67].

British fossils from the Old Red Sandstone from the north of Scotland, Orkney and Caithness, originally figured by Hugh Miller and compared by him with algae but more especially with recent Lycopods, were subsequently placed by Carruthers[68] in the genus Psilophyton as P. Dechianum, the specific designation being chosen on the ground that the Scotch specimens are specifically identical with fossils described by Goeppert[69] as Haliserites Dechianus.

Various opinions have been expressed in regard to the nature of the Devonian species Haliserites Dechianus Goepp. with which Carruthers[70] identified Miller’s Old Red Sandstone plant: reference may be made to a paper by White[71] containing figures of dichotomously branched impressions described as species of Thamnocladus which he includes among the algae.

In describing some Belgian impressions of Devonian age as Lepidodendron gaspianum Daws. Crépin[72] states that Carruthers has come to regard the specimens named by him Psilophyton Dechianum as branches of a Lepidodendron; he also quotes Carruthers as having expressed the opinion that the name Psilophyton had been employed by Dawson for two kinds of fossils, some being twigs of Lepidodendron while others, identified by Dawson as the reproductive branches of species of Psilophyton, represent the spore-cases of ferns comparable with Stur’s genus Rhodea[73]. One of the examples figured by Carruthers[74] as P. Dechianum from Thurso (preserved in the British Museum, no. 52636), measuring 34 cm. in length and 288 mm. broad, bears a close resemblance to a fern rhizome covered with ramental scales such as that of a species of Davallia. Other Belgian specimens described by Gilkinet[75] as Lepidodendron burnotense, like Crépin’s species, are no doubt generically identical with some of the Scotch and Canadian fossils placed in the genus Psilophyton, though Penhallow[76] considers that the species Lycopodites Milleri is more correctly referred to Lycopodites than to Psilophyton.

A more recent paper on the Geology of the Perry basin in South-eastern Maine by Smith and White[77] contains a critical summary of the literature on Psilophyton and drawings of specimens. The latter afford good examples of Pre-Carboniferous plant fragments, such as are often met with in various parts of the world, which conform in habit to the New Brunswick specimens made by Dawson the type of his genus.

An examination of material in the Montreal Museum and of Hugh Miller’s specimens in the Edinburgh collection leads me to share the opinion of Count Solms-Laubach that the name Psilophyton has been applied to plants which should not be included under one generic title. As Kidston[78] pointed out, the Canadian species Psilophyton robustius is not generically distinct from British and Belgian specimens referred to Lepidodendron; it may possibly be identical with the Bohemian plants on which Stur founded his genus Hostinella[79]. The Devonian plants described by Stur have since been examined by Jahn[80] who regards them as vascular plants, and not as algae to which Stur referred them; he mentions two species of Psilophyton but gives no figures.

The “spore-cases” of Dawson may be found to be the microsporangia or perhaps the small seeds of some pteridosperm; the forked axes with a smooth surface and others figured by Miller and by Dawson, with the surface covered with scales suggesting the ramenta of a fern, may be the rachises or rhizomes of filicinean plants. Other specimens may be Lepidodendron twigs, as for example the petrified fragments figured by 29Dawson as Psilophyton princeps; while the stem identified as P. robustius is most probably that of a Gymnosperm. It is doubtful whether a useful purpose is served by retaining the genus Psilophyton. It was in the first instance instituted on the assumption, which cannot be upheld, that the abundant material in the New Brunswick beds bore a sufficiently close resemblance to the rhizome and aerial branches of Psilotum. Psilophyton has served as a name for miscellaneous plant fragments, many of which are indeterminable. Dr White concludes his account of the genus with the following words[81]:

“The examination of such so-called Psilophyton material as I have seen shows the existence in America of two or more groups, represented by several fairly well-marked species which possess stratigraphical value, and which should be carefully diagnosed and illustrated. It is probable also that additional material throwing light on the structure and relationships of these very remarkable early types of land-plants will be discovered at some locality. The inspection of the material in hand emphasises the need, as was pointed out by Solms-Laubach, for the revision of the material referred by various authors to Psilophyton, together with a thorough re-examination and re-publication of the types.”

Until a thorough re-examination has been made of the Canadian material, with a view to determine whether there exist substantial reasons for the retention of Dawson’s genus, it is undesirable to continue to make use of this name for Pre-Carboniferous fossils which are too incomplete to be assigned with certainty to a definite group of plants. Dr White draws attention to the similarity of some of the Perry basin specimens to Nathorst’s genus Cephalotheca[82] from Devonian rocks of Bear Island in the Arctic regions, a comparison which might be extended to other genera and which serves to illustrate the possibility that many of the specimens labelled Psilophyton may eventually be recognised as examples of well defined generic types belonging to more than one group of plants.

LYCOPODIALES.

The recent members of the Lycopodiales are considered apart from the extinct genera in order that our examination of the latter may be facilitated by a knowledge of the salient characteristics of the surviving types of this important section of the Pteridophyta. A general acquaintance with the extinct as well as with the recent genera will enable us to appreciate the contrasts between the living and the fossil forms and to realise the prominent position occupied by this group in the Palaeozoic period, a position in striking contrast to the part played by the diminutive survivors in the vegetation of the present day. In the account of the recent genera special attention is drawn to such features as afford a clue to the interpretation of the fossils, and the point of view adopted, which at times may appear to lead to an excessive attention to details, is necessarily somewhat different from that represented in botanical text-books[83].

The existing plants included in the Lycopodiales are in nearly all cases perennial herbaceous pteridophytes, exhibiting 31in their life-histories a well marked alternation of generations. The sporophyte (asexual generation) is characterised by the relatively small size of the leaves except in the genus Isoetes (fig. 132) and in the Australian and New Zealand genus Phylloglossum. The stems are usually erect or trailing, pendulous in epiphytic species or small and tuberous in Isoetes and Phylloglossum. The repeated forking of the shoots (monopodial and dichotomous branching) is a prominent feature of the group. The vascular tissue of the stem usually assumes the form of a single axial strand (stele) (fig. 125), but the shoots of some species of Selaginella often contain two or more distinct steles (fig. 131). The group as a whole is characterised by the centripetal development of the xylem composed almost entirely of scalariform tracheids: secondary xylem and phloem of a peculiar type occur in Isoetes, and the production of secondary xylem elements in a very slight degree has been noticed in one species of Selaginella (S. spinosa)[84]. The roots are constructed on a simple plan, having in most cases only one strand of spiral protoxylem elements (monarch structure). In Lycopodium, in which stem and root anatomy are more nearly of the same type than in the majority of plants, several protoxylem strands may be present. The sporangia are axillary or, more frequently, borne on the upper surface of sporophylls, which are either identical with or more or less distinct from the foliage leaves; in the latter case the sporophylls often occur in the form of a well defined strobilus (cone) at the tips of branches.

The gametophyte (sexual generation) is represented by prothalli which, in the homosporous genera, may live underground as saprophytes, or the upper portion may develop chlorophyll and project above the surface of the ground as an irregularly lobed green structure (e.g. Lycopodium cernuum)[85]. In the heterosporous forms the prothalli are much reduced and do not lead an independent existence outside the spore by the membrane of which they are always more or less enclosed. The sexual organs are represented by antheridia and archegonia; 32the male cells are provided with two cilia except in Isoetes which has multiciliate antherozoids like those of the ferns.

The existing Lycopods, though widely distributed, never grow in sufficiently dense masses to the exclusion of other plants to form a conspicuous feature in the vegetation of a country. The inconspicuous rôle which they play among the plant-associations of the present era affords a striking contrast to the abundance of the arborescent species in the Palaeozoic forests of the northern hemisphere.

Lycopodiaceae. Lycopodium, represented by nearly 100 species, forms a constituent of most floras: epiphytic species predominate in tropical regions, while others flourish on the mountains and moorlands of Britain and in other extra-tropical countries. For the most part Lycopodium exhibits a preference for a moist climate and appears to be well adapted to habitats where the amount of sunlight is relatively small and the conditions of life unfavourable for dense vegetation. Mountains and islands constantly recur as situations from which species have been recorded. Some species are essentially swamp-plants, e.g. Lycopodium inundatum, a British species, and L. cruentum from the marshes of Sierra Nevada. A variety of the American species, L. alopecuroides (var. aquaticum) affords an instance of a submerged form, which has been collected from an altitude of 12–14,000 ft. on the Andes and Himalayas. It is noteworthy that a considerable variety of habitats is represented within the limits of the genus and that many species are sufficiently hardy to exist in circumstances which would be intolerable to the majority of flowering plants[86].

The British species frequently spoken of as Club Mosses, include Lycopodium Selago, L. annotinum, L. clavatum, L. alpinum, and L. inundatum.

Selaginellaceae. The species of Selaginella, over 300 in number, are widely spread in tropical and subtropical forests, growing on the ground with trailing, suberect or erect stems climbing over taller and stouter plants or as pendulous epiphytes on forest trees.

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Selaginella lepidophylla, a tropical American type, popularly known as the Resurrection plant, and often erroneously spoken of as the Rose of Jericho[87], possesses the power of rolling up its shoots during periods of drought and furnishes an example of a species adapted to conditions in marked contrast to those which are most favourable to the majority of species.

The only British species is Selaginella spinosa named by Linnaeus Lycopodium selaginoides and occasionally referred to as Selaginella spinulosa A. Br. (not to be confounded with a Javan species S. spinulosa Spring[88]).

Isoetaceae. Isoetes (fig. 132), of which Mr Baker in his Handbook of the Fern-Allies enumerates 49 species, is a type apart, differing in habit as in certain other characters from the other members of the Lycopodiales. Some botanists[89] prefer to include the genus among the Filicales, but the balance of evidence, including resemblances between Isoetes and extinct Lycopodiaceous plants, would seem to favour its retention as an aberrant genus of the group Lycopodiales. Some species are permanently submerged, others occur in situations intermittently covered with water, and a few grow in damp soil. Isoetes lacustris is found in mountain tarns and lakes of Britain and elsewhere in Central and Northern Europe and North America. Isoetes hystrix[90], a land-form occurs in Guernsey, North-East France, Spain and Asia Minor.

Lycopodiaceae.

The monotypic genus Phylloglossum, represented by P. Drummondii of Australia and New Zealand, though interesting from the point of view of its probable claim to be considered the most primitive type of existing Lycopodiaceous plants, need not be dealt with in detail. A complete individual, which does not exceed 4 or 5 cm. in length, consists of a very small tubercle or protocorm bearing a rosette of slender subulate leaves and prolonged distally as a simple naked axis which overtops the foliage leaves and terminates in a compact cluster of 34small scale-like sporophylls, each subtending a single sporangium[91].

Lycopodium. It would be out of place in a volume devoted mainly to fossil plants to attempt a comprehensive account of the general morphology of recent species, and indeed our knowledge of the anatomical characters of the genus is still somewhat meagre. For purposes of comparison with extinct types, it is essential that some of the more important morphological features of existing species should be briefly considered. The additions made to our knowledge of the gameophyte[92] of European and tropical species during the last two decades have revealed a striking diversity in habit.

In several species, grouped round the widely distributed type Lycopodium Selago Linn., the comparatively short, erect or suberect, shoots form fairly compact tufts; the ordinary foliage-leaves function as sporophylls, and the sporangia are not localised on special portions of shoots. From this type, we pass to others in which the fertile leaves tend to be confined to the tips of branches, but hardly differ in form from the sterile. A further degree of specialisation is exhibited by species with well-defined cones composed of leaves (or bracts), the primary function of which is to bear sporangia and to afford a protective covering to the strobilus[93].

Lycopodium rufescens Hook. An Andian species with stout dichotomously branched erect stems bears on the younger shoots crowded leaves with their thick and broadly triangular laminae pointing upwards, but on the older and thick shoots the laminae are strongly reflexed (fig. 121, A). The lower part of the specimen represented in fig. 121, A, shows tangentially elongated scars and persistent leaf-bases or cushions left on the stem after the removal of the free portions of the leathery leaves, a surface-feature which also characterises the Palaeozoic genus Lepidodendron. The reflexed leaves and persistent leaf-cushions are clearly seen in the piece of old stem of Lycopodium dichotomum Jacq., a tropical American species 36reproduced in fig. 121, B. Such species as L. erythraeum Spring, and others with stiff lanceolate leaves exhibit a striking resemblance to the more slender shoots of some recent conifers, more especially Araucaria excelsa, A. Balansae, Cryptomeria, Dacrydium and other genera.

Fig. 121. Lycopodium.

1. Lycopodium rufescens.
2. L. dichotomum.
3. L. tetragonum.
4. L. nummularifolium.
5. L. Dalhousianum.
6. L. casuarinoides.
7. L. volubile.

(From specimens in the Cambridge Herbarium and Botanic Garden. M.S.)

Fig. 122. Lycopodium squarrosum. The branches of the larger shoot terminate in cones. (From a plant in the Cambridge Botanic Garden. Reduced.)

In Lycopodium tetragonum Hook., (fig. 121, C), a species from the Alpine region of the Andes, the long, pendulous and repeatedly forked branches bear four rows of fleshy ovate leaves and simulate the vegetative characters of certain conifers.

37

Fig. 123. Lycopodium cernuum.
(From a specimen in the Cambridge Herbarium. ½ nat. size.)

38

L. squarrosum Forst. (fig. 122) a tropical species from India, Polynesia, and other regions, is characterised by its stout stems reaching a diameter of 2·5 cm., bearing long pendulous branches with large terminal cones composed of sporophylls differing but slightly from the foliage leaves. The plant represented in the photograph serves as a good illustration of the practical identity in habit between Palaeozoic and recent genera.

Fig. 124. Lycopodium obscurum.

L. Dalhousianum Spring, from the mountains of the Malay Peninsula and Borneo, has larger leaves of finer texture with a distinct midrib reaching a length of 2–3 cm. (fig. 121, E). Another type is illustrated by L. nummularifolium Blume, also a Malayan species, in which the leaves are shorter, broadly oblong or suborbicular, and the branches terminate in narrow and often very long strobili (sometimes reaching a length of39 30 cm.) with small bracts in striking contrast to the foliage leaves (fig. 121, D). A similar form of long and slender strobilus occurs in L. Phlegmaria Linn., a common tropical Lycopod: the frequent forking of the strobili noticed in this and other species is a character not unknown among fossil cones (Lepidostrobi).

L. cernuum Linn. (fig. 123), another widely spread tropical type, offers an even closer resemblance than L. squarrosum to the fossil Lepidodendra. The stiff erect stem, reaching in some cases a length of several feet, bears numerous repeatedly forked branches, with crowded linear leaves, terminating in short cylindrical cones with broadly ovate sporophylls. A similar habit characterises the North American species L. obscurum Linn. (fig. 124) bearing cones several centimetres in length.

L. casuarinoides Spring (fig. 121, F) an eastern tropical species, is worthy of notice as exhibiting a peculiar form of leaf consisting of a very small lamina, 3 mm. in length, borne on the top of a long decurrent base, which forms a narrow type of leaf-cushion, bearing some resemblance to the long and rib-like cushions of certain species of Sigillaria, and recalling the habit of slender fossil twigs referred to the Coniferae under such names as Widdringtonites, Cyparissidium, Sphenolepidium.

L. volubile Forst. (fig. 121, G) a New Zealand species, in habit and leaf-form bears a close resemblance to the Jurassic Lycopodites falcatus Lind. and Hutt. (fig. 137): it is also a representative of a few species of Lycopodium which agree with the majority of species of Selaginella in having two kinds of sterile leaves, comparatively long falcate leaves forming two lateral rows and smaller appressed leaves on the upper surface of the branches.

These examples suffice to illustrate the general appearance presented by the vegetative shoots of recent species of which the foliage leaves vary considerably—from the small scale-leaves of Lycopodium tetragonum, to the very slender linear subulate leaves of such a species as L. verticillatum Linn. or the long and broader lamina of L. Dalhousianum (fig. 121, E). It is obvious that fragments of the various types preserved as fossils might well be mistaken either for some of the larger mosses or40 for twigs of conifers. As Dr Bommer[94] has pointed out in his interesting paper on “Les causes d’erreur dans l’étude des empreintes végétales” some dicotyledonous plants may also simulate the habit of Lycopods: he cites Phyllachne clavigera Hook (Candolleaceae), Tafalla graveolens Wedd (Compositae) and Lavoisiera lycopodioides Gard. (Melastomataceae). Another point illustrated by fig. 121 is the close agreement in habit and in the form of the leaves and leaf-cushions between the recent plants and the Palaeozoic Lepidodendreae.

In his masterly essay “On the vegetation of the Carboniferous Period, as compared with that of the present day” Sir Joseph Hooker called attention to the variation in the shape and arrangement of the leaves in the same species of Lycopodium. The three woodcuts which he publishes of Lycopodium densum, a New Zealand species, afford striking examples of the diversity in habit and leaf-form and justify his warning “that if the species of Lepidodendron were as prone to vary in the foliage as are those of Lycopodium, our available means for distinguishing them are wholly insufficient[95].”

As we have already noticed, there is a considerable diversity among recent species, both as regards habitat and habit; in the anatomy of the stem also corresponding variations occur within the limits of a well-defined generic type of stele. In species with creeping stems, such as L. clavatum[96], the stele exhibits an arrangement of vascular tissue characteristic of the plagiotropic forms. The xylem consists of more or less horizontal plates of scalariform tracheae, each surrounded by small-celled parenchyma, alternating with bands or groups of somewhat ill-defined phloem. The protoxylem and protophloem elements occupy an external position (exarch), pointing to a centripetal development of the metaxylem. This centripetal or root-like character of the primary xylem is an important feature in recent as in fossil Lycopods. The close agreement between the roots and stems of recent species in the disposition of the vascular elements also denotes a simpler type of anatomy 41than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. A pericycle, 2–6 cells in breadth, encloses the xylem and phloem bands and this is succeeded by an endodermis, 2–3 cells broad, with vaguely defined limits. In L. clavatum, as in L. alpinum, another British species, the broad cortex is differentiated into42 three fairly distinct regions; abutting on the endodermis is a zone several layers broad of thick-walled cells constituting an inner cortex modified for protection and support; the central region consists of larger and thinner-walled cells adapted for water-storage and aeration; beyond this is an outer cortical zone of firmer and thicker elements. The prominent leaf-bases or leaf-cushions (fig. 125, A, lc) give to the surface of a transverse section a characteristic appearance which presents the closest agreement with that of the younger shoots of Lepidodendron. From the peripheral protoxylem groups small strands of xylem are given off, which follow a steeply ascending course through the cortex to the single-veined leaves. The leaf-traces, in several species at least, are characterised by a mesarch structure (fig. 125, F, G), the spiral protoxylem elements occupying an approximately central position. The mesophyll of the leaves varies in regard to the extent of differentiation into a palisade and spongy parenchyma; in all cases there is a single vascular bundle occasionally accompanied by a secretory duct.

Fig. 125.

1. Lycopodium dichotomum. Transverse section of stem: lc, leaf-cushion; lt, leaf-trace; R, roots.
2. L. cernuum, portion of cortex of fig. H, enlarged.
3. L. saururus. Cortex: lt, leaf-trace; a, thin-walled tissue; b, thick-walled tissue; lc, lacuna.
4. L. saururus. Stele: x, xylem; p, phloem.
5. Portion of fig. D, enlarged: px, protoxylem; p, phloem.
6. Transverse section of leaf of Lycopodium.
7. Vascular bundle of leaf: px, protoxylem.
8. L. cernuum: b, branch of stele; c–c″, cortex; s, space in cortex; lt, leaf-trace.
9. Stele of fig. H, enlarged (phloem omitted).

In erect stems of Lycopodium, as represented by L. cernuum (figs. 123, 125, H, I), L. Dalhousianum, L. squarrosum (fig. 122) and many others, the stele presents a characteristic appearance due to the xylem plates being broken up into detached groups or short uniseriate bands with the interspaces occupied by phloem islands. This type of structure bears a superficial resemblance to that in the single stele of certain species of the fern Lygodium[97], but it is distinguished by the islands of phloem scattered through the stele. In other species the xylem tends to assume the form of a Maltese cross (e.g. L. serratum Thbg.) or it may be disposed as V-shaped and sinuous bands terminating in broad truncate ends composed of protoxylem elements. This form of the xylem and the distribution of the phloem groups are shown in fig. 125, D, E, drawn from a section of a plant of Lycopodium saururus Lam.[98] collected by Mr A. W. Hill at an altitude of 15,000 feet on the Andes of Peru. The position of the protoxylem is shown fig. 125, E, px.

43

While several species possess a cortex of three distinct zones (fig. 125, H, c, c′, c″), in others the extra-stelar tissue is much more homogeneous, consisting of thin-walled parenchyma or in some cases of thick-walled elements; as a general rule, however, there is a tendency towards a more compact arrangement in the inner and outer portions of the cortex as contrasted with the larger and more loosely connected cells of the middle region. In certain types the middle cortex contains fairly large spaces, as in the swamp-species L. inundatum, which with L. alopecuroides exhibits another feature of some interest first described by Hegelmaier[99]. If a transverse section of the stem of L. inundatum be examined the leaf-traces are seen to be accompanied by a circular canal containing mucilage which extends into the lamina of the leaf. In a specimen of L. cernuum[100] obtained at a height of 2500 ft. by Professor Stanley Gardiner in the Fiji Islands, the leaf-traces (fig. 125, B lt) were found to be accompanied for part of their course by a well-marked secretory space (fig. 125, B, s). There is little doubt that the presence of these mucilage canals is directly connected with a certain type of habitat[101] and attention is called to them in view of a resemblance which they offer to a characteristic strand of tissue, known as the parichnos, which is associated with the leaf-traces of Lepidodendreae and Sigillarieae. In the section shown in fig. 125, H, the xylem of the stele forms more continuous bands than is often the case in L. cernuum which has already been described as having its xylem in small detached groups. The presence of the smaller branch-stele (fig. 125, H, b) affords an example of monopodial branching. The outer cortex of L. saururus (fig. 125, C) exhibits a somewhat unusual feature in the distribution of the thicker-walled tissue (b) which encloses a patch of more delicate parenchyma (a) with large lacunae (lc) in the region of the 44leaf-bases, and presents the appearance of an irregular reticulum. This arrangement of the mechanical tissue in the outer cortex is comparable with that in stems of some species of Sigillaria.

In certain species of Lycopodium the roots[102], which arise endogenously from the axial vascular cylinder, instead of passing through the cortex of the stem by the shortest route, bend downwards and bore their way in a more or less vertical direction before emerging at or near the base of the aerial shoot. The transverse section of L. dichotomum represented in fig. 125, A, shows several roots (R) in the cortex; they consist of a xylem strand of circular or crescentric form accompanied by phloem and enclosed by several layers of root-cortex. The roots of Lycopodium do not always present so simple a structure as those of L. dichotomum; the xylem may have an irregularly stellate form with as many as ten protoxylem groups.

Reproductive Shoots[103]. In Lycopodium Selago the foliage leaves serve also as sporophylls and, as Professor Bower[104] has pointed out, the branches exhibit to some extent a zonal alternation of sterile and fertile leaves; in other species, in which foliage leaves and sporophylls are practically identical, the sporangia occur sporadically on the ordinary leaves. In species with well-defined terminal cones the lower sporophylls may bear arrested sporangia and thus form transitional stages between sterile and fertile leaves, a feature which occurs also in the male and female flowers of many recent Araucarieae[105]. The sporangia[106] (fig. 126, D, F) are usually reniform and compressed in a direction parallel to the surface of the cone-scales; they are developed from the upper surface and close to the base of the fertile leaf to which they are attached by a short and thick stalk (e.g. L. inundatum) or by a longer and more slender pedicel (L. Phlegmaria, fig. 126, E). On maturity the sporangia open as two valves in the plane of compression 45and the line of dehiscence is determined in some species at least by the occurrence of smaller cells in the wall. In transverse sections of cones in which the sporangia are strongly saddle-shaped, the sporophylls may appear to bear two sporangia.46 This is well shown in the section of a cone of L. clavatum shown in fig. 126, F. The sporangia a and b are cut through in an approximately median plane showing the irregular outline of the sterile pad (p) of tissue in the sporogenous cavity. Those at c and d have been traversed at a lower level and the two lobes of the saddle-shaped sporangia are cut below the attachment to the sporophyll. The distal laminae of the sporophylls, cut at different levels, are seen at the periphery of the cone.

Fig. 126.

1. Lycopodium cernuum, longitudinal section of strobilus; a, band of lignified cells.
2. L. cernuum. Cell from sporangium wall.
3. L. cernuum. Sporophyll and sporangium; lt, vascular bundle.
4. L. clavatum. Part of radial longitudinal section of strobilus; p, sterile tissue.
5. L. Phlegmaria. Sporophyll and stalked sporangium.
6. L. clavatum. Transverse section of strobilus; p, sterile pad.

In longitudinal radial section of some cones the sporangia appear to occupy an axillary position, but in others (e.g. L. clavatum) they are attached to the horizontal portion of the sporophyll almost midway between the axis of the cone and the upturned distal end of the sporophyll (fig. 126, D). The wall of a sporangium frequently consists of 2–3 cell-layers and in some cases (e.g. L. dichotomum), it may reach a thickness of seven layers, resembling in this respect the more bulky sporangia of a certain type of Lepidodendroid cone. The sporogenous tissue is separated from the stalk of the sporangium by a mass of parenchymatous tissue which may project as a prominent pad (fig. 126, D, F, p) into the interior of the sporogenous cavity. This basal tissue (the subarchesporial pad of Bower[107]) has been observed in L. clavatum to send up irregular processes of sterile cells among the developing spores, suggesting a comparison with the trabeculae which form a characteristic feature of the sporangia of Isoetes and with similar sterile strands noticed by Bower[108] in Lepidostrobus (cone of Lepidodendron).

Each sporophyll is supplied by a single vascular bundle which according to published statements never sends a branch to the sporangium base. The fertile tips of the foliage shoots of L. cernuum (figs. 126, A–C) afford good examples of specialised cones. The surface of the cone is covered by the broadly triangular laminae of sporophylls (fig. 126, C) which in their fimbriate margins resemble the Palaeozoic cone-scales described by Dr Kidston[109] as Lepidostrobus fimbriatus. The distal portions of the sporophylls are prolonged downwards 47(fig. 126, A) to afford protection to the lower sporangia, their efficiency being increased by the lignified and thicker walls (A, a) of the cells in the lower portion of the laminar expansion. The cells of the sporangial wall are provided with strengthening bands which in surface-view (fig. 126, B) present the appearance of prominent pegs. Since the appearance of Miss Sykes’s paper on the sporangium-bearing organs of the Lycopodiaceae, Dr Lang[110] has published a more complete account of the structure of the strobilus of Lycopodium cernuum in which he records certain features of special interest. The importance of these morphological characters is increased by their agreement, as shown by Lang, with those of the Palaeozoic cone Spencerites[111]. The sporophylls of a cone (12 mm. long by 3 mm. in diameter) of Lycopodium cernuum show an abrupt transition from the foliage leaves, but like these they occur in alternate whorls of five. A large sporangium is attached to the upper face of each sporophyll close to the base of the obliquely vertical distal lamina 48(fig. 127); each sporophyll, which is supplied with a single vascular bundle, has a large mucilage-cavity (m) in its lower region. “The mucilaginous change” in the sub-sporangial portion of a sporophyll “extends to the surface involving the epidermis, so that this portion of the sporophyll-base may be described as consisting of a mass of mucilage bounded below by a structureless membrane[112].” Dehiscence of the sporangia occurs at the middle of the distal face (fig. 127, x). As seen in the radial section (fig. 127, ma) the outer margin of the base of the sporophyll bears a short outgrowth. The leaf-bases of each whorl hang down between the sporangia of the alternating whorl below, and the base of each sporophyll is coherent with the margins of the two sporophylls of the next lower whorl between which it lies, the sporangia being thus closely packed and lying in a pocket “open only on the outer surface of the cone.” Fig. 128 represents a transverse section through a cone in the plane AA of fig. 127; this traverses the sporangia and their subtending bracts (b) of one whorl and the dependent bases of the sporophylls of the next higher whorl in the region of the mucilage-sacs (m), which are bounded at the periphery by the 49outer tissue of the sporophylls (a). A transverse section in the plane BB of fig. 127 is shown in fig. 129: the pedicels and a part of each vascular strand are seen at b radiating from the axis of the cone; one sporophyll (sp, a) is cut through in the region of the pad of tracheal tissue that characterises the short sporangial stalks. The upper portions of the sporangia of the next lower whorl, which project upwards against the mucilaginous bases of the sporophylls above (cf. fig. 127, BB) are shown at c and external to them, at a, the section has cut through the outer persistent portions of these sporophyll bases.

Fig. 127. Radial longitudinal section of the cone of Lycopodium cernuum. (After Lang.)

Fig. 128. Transverse section of the cone of Lycopodium cernuum, in its plane AA of fig. 127. (After Lang.)

Fig. 129. Transverse section of the cone of Lycopodium cernuum in the plane BB of fig. 127. (After Lang.)

As Lang points out, this highly complex structure is an expression of the complete protection afforded to the sporangia of a plant met with in exposed situations in the tropics; it is also of importance from a morphological standpoint as exhibiting an agreement with the extinct type of Lycopod cone represented by Spencerites.

Selaginellaceae.

Selaginella differs from Lycopodium in the production of two kinds of spores, megaspores and microspores, and, in the50 great majority of species, in the dimorphic character of the foliage leaves, which are usually arranged in four rows, the laminae of the upper rows being very much smaller than those of the lower (fig. 130, 1–3). The smaller leaves are shown more clearly in fig. 130, 1a. It is obvious from an examination of a Selaginella shoot, such as is shown in fig. 130, that in fossil specimens it would often be almost impossible to recognise the existence of two kinds of leaves. Some species, e.g. Selaginella spinosa[113], the sole British representative of the genus, are homophyllous and agree in this respect with most species of Lycopodium. Another feature characteristic of Selaginella, as 51contrasted with Lycopodium, is the presence of a ligule in both foliage leaves and sporophylls. This is a colourless thin lamina attached by a comparatively stout foot to the base of a pit on the upper surface and close to the lower edge of the leaf (fig. 130, 4, l; fig. 131, E, F, l).

Fig. 130. Selaginella grandis. (1–3, nat. size.)

In an erect species, such as S. grandis Moore[114] (fig. 130 and fig. 131, G) from Borneo, the main shoots, which may attain a height of 2–3 feet, bear small and inconspicuous leaves of one kind, but the lateral and repeatedly forked shoots are heterophyllous. The passage from the homophyllous to the heterophyllous arrangement is shown in the transition from the erect to the dorsiventral habit of the lateral shoots (fig. 130, 2). The monopodially or dichotomously branched shoots produce long naked axes at the forks; these grow downwards to the ground where they develop numerous dichotomously forked branches. For certain reasons these naked aerial axes were named rhizophores and have always been styled shoots, the term root being restricted to repeatedly forked branches which the rhizophores produce in the soil. It has, however, been shown by Professor Harvey-Gibson[115] that there is no sufficient reason for drawing any morphological distinction between rhizophores and roots, the term root being applicable to both.

Our knowledge of the anatomy of Selaginella, thanks chiefly to the researches of Harvey-Gibson[116], is much more complete than in the case of Lycopodium. The stems, which may be either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the same axis in different regions may exhibit considerable variation in the structure and arrangement of the vascular tissue. In the well-known species, Selaginella Martensii, the stem, which is partly trailing, partly ascending, possesses a single ribbon-shaped stele composed of scalariform tracheids with two marginal protoxylems formed by the fusion of the leaf-traces of the dorsal and ventral leaves respectively. As in Lycopodium the metaxylem tracheae are as a rule scalariform, but reticulate xylem elements are by no means unknown. The tracheal band, 53surrounded by parenchymatous elements, is enclosed by phloem with external protophloem elements. The characteristic features of the stele are shown in the diagrammatic drawing of a section of another species—S. Willdenowii—represented in fig. 131, A.

Fig. 131.

1. Selaginella Willdenowii. Transverse section of stem: a, outer cortex; p, phloem; t, trabeculae.
2. S. spinosa, stem: px, protoxylem.
3. S. laevigata var. Lyallii, section of stele: t, ridge of xylem cylinder; e, endodermis.
4. S. rupestris, seedlings with cotyledons (c) protruding beyond the sporophylls (b).
5. Transverse section of Selaginella leaf-base: l, ligule; lt, leaf-trace.
6. Portion of G. enlarged.
7. S. grandis. Longitudinal section of strobilus: bb, sporophyll-trace; l, ligule.

(A, B, C, E, F, after Harvey-Gibson; D, after Miss Lyon.)

A pericycle composed of one or two layers of chlorophyll-containing cells encircles the whole stele which is suspended in a lacuna by trabeculae (fig. 131, A, B, t) connecting the pericycle with the inner edge of the broad cortex. The trabeculae consist in part of endodermal cells characterised by cuticular bands. The cortex is usually differentiated into three fairly distinct regions. Mechanical tissue of thick-walled fibres constitutes the outer region (a); the middle cortex consists of thinner-walled parenchyma, the elements of which become smaller and rather more compactly arranged in the inner zone. The middle cortex is frequently characterised by the presence of spaces and by the hyphal or trabecular structure of the tissue, a feature which, as Bower[117] pointed out, is common to many recent and fossil members of the Lycopodiales. In some cases, e.g. S. erythropus, from tropical America, the cortex of the creeping stem consists entirely of thick-walled cells. Selaginella grandis (fig. 130) has “a short decumbent stem rooted at close intervals[118],” from which thick erect aerial shoots rise to a height of one foot or more. In the apical region these erect axes give off repeatedly forked foliage shoots on which the spiral phyllotaxis of the homophyllous axis is gradually replaced by four rows of two kinds of leaves (fig. 130, 2). The anatomy of this species agrees with that of S. Martensii. The trailing or semi-erect and homophyllous shoots of Selaginella spinosa[119] present a distinct type of vascular anatomy. The upper part of the ascending stem has an axial strand of xylem with seven peripheral groups of spiral protoxylem tracheae (fig. 131, B); in the trailing portion of the shoot the protoxylem elements occur as one central group in the solid rod of metaxylem through which the leaf-traces pass on their way to the axial protoxylem. This type is important 54as affording an exception, in the endarch structure of the xylem, to the usual exarch plan of the stelar tissues. This species is the only one in which any indication of the production of secondary xylem elements has so far been recorded. Bruchmann[120] has shown that, in the small tuberous swelling which occurs at the base of the young shoot (hypocotyl), a meristematic zone is formed round the axial vascular strand and by its activity a few secondary tracheids are added to the primary xylem. With this exception Selaginella appears to have lost the power of secondary thickening, the possession of which constitutes so striking a feature of the Palaeozoic Lycopods. Another type is represented by S. inaequalifolia, an Indian species, the shoots of which may have either a single stele or as many as five, each in its separate lacuna. The homophyllous S. laevigata var. Lyallii Spr., a Madagascan species, affords a further illustration of the variation in plan of the vascular tissues within the genus. There is a considerable difference in structure between the erect and creeping shoots; in the former there may be as many as 12–13 steles, which gradually coalesce before the vertical axis joins the creeping rhizome to form one central and four peripheral steles. In the rhizome there is usually a distinct axial stele without protoxylem, surrounded by an ill-defined lacuna and enclosed by a cylindrical stele (solenostele)[121] usually two tracheae in width with four protoxylem strands on its outer edge. The continuity of the tubular stele is broken and, in transverse section, it assumes the form of a horse-shoe close to the base of an erect shoot to which a crescentic vascular strand is given off. Harvey-Gibson[122] has figured a section of the rhizome of this type in which the axial vascular strand is represented by a slight ridge of tracheae (fig. 131, C, t) projecting towards the 55centre of the axis of the tubular stele. The cylindrical stele consists of xylem with external and internal phloem (p): cuticularised endodermal cells occur at e and e.

Reference has already been made to the descending naked branches given off from the points of ramification of the foliage shoots of Selaginella. It has been shown by Harvey-Gibson[123] that these branches, originally designated rhizophores by Nägeli and Leitgeb, as well as the dichotomously branched roots which they produce below the level of the ground, possess a single vascular strand of monarch type. It is interesting to find that in some species the aerial portion of the rhizophore has a xylem strand with a central protoxylem, an instance of endarch structure like that in certain portions of the shoot-system of S. spinosa. The root-anatomy of Selaginella and the dichotomous habit of branching afford points of agreement with the subterranean organs of Lepidodendron and Sigillaria.

Leaves. The leaves of Selaginella[124] usually consist of a reticulum of loosely arranged cells, but in some cases part of the mesophyll assumes the palisade form. The single vascular bundle consists of a few small annular or spiral tracheae and at the apex of the lamina the protoxylem elements are accompanied by several short reticulated pitted elements. Both foliage leaves and sporophylls are characterised by the possession of a ligule, a structure which may present the appearance of a somewhat rectangular plate (fig. 130, 4, l, and fig. 131, E–G, l) or assume a fan-shaped form with a lobed or papillate margin. The base, composed of large cells, is sunk in the tissue of the leaf close to its insertion on the stem (fig. 131, E, l) and enclosed by a well-marked parenchymatous sheath. The sheath is separated from the vascular bundle of the leaf by one or more layers of cells, and in some species these become transformed into short tracheids. The ligule is regarded by Harvey-Gibson[125] as a specialised ramentum which serves the temporary function of keeping moist the growing-point and young leaves.

Cones. The terminal portions of the branches of Selaginella usually bear smaller leaves of uniform size which function as 56sporophylls, but in this genus the fertile shoots do not generally form such distinct cones as in many species of Lycopodium. In S. grandis (figs. 130, 3; 131, G) the long and narrow strobili consist of a slender axis bearing imbricate sporophylls in four rows: each sporophyll subtends a sporangium situated between the ligule and the axis of the shoot. The sporangium may be developed from the axis of the cone or, as in Lycopodium, from the cells of the sporophyll[126]. In some species the lower sporophylls bear only megasporangia, each normally containing four megaspores, the microsporangia being confined to the upper part of the cone. This distribution of the two kinds of sporangia is, however, by no means constant[127]: in some cases, e.g. S. rupestris, cones may bear megasporangia only, and in the cone of S. grandis, of which a small piece is represented in fig. 131, G, all the sporangia were found to contain microspores.

The occurrence of two kinds of spores in Selaginella constitutes a feature of special importance from the point of view of the relationship between the Phanerogams, in which heterospory is a constant character, and the heterosporous Pteridophytes. One of the most striking distinctions between the Phanerogams and the rest of the vegetable kingdom lies in the production of seeds. Recent work has, however, shown that seed-production can no longer be regarded as a distinguishing feature of the Gymnosperms and Angiosperms. Palaeozoic plants which combined filicinean and cycadean features resembled the existing Phanerogams in the possession of highly specialised seeds. This discovery adds point to the comparison of the true seed with structures concerned with reproduction in seedless plants, which in the course of evolution gave rise to the more efficient arrangement for the nursing, protection, and ultimate dispersal of the embryo. In the megaspore of Selaginella we have, as Hofmeister was the first to recognise in 1851, a structure homologous with the embryo-sac of the Phanerogam. The embryo-sac consists of a large cell produced in a mass of parenchymatous tissue known as the nucellus which is almost completely enclosed by one or more integuments. Fertilisation of the egg-cell within the embryo-sac 57takes place as a rule while the female reproductive organ is still attached to the parent-plant and separation does not occur until the ovule has become the seed.

In a few cases, notably in certain plants characteristic of Mangrove swamps, continuity between the seed and its parent is retained until after germination. The megasporangium of Selaginella dehisces[128] along a line marked out by the occurrence of smaller cells over the crest of the wall. It has been customary to describe the megaspores as being fertilised after ejection from the sporangia. This earlier separation from the parent and the absence of any protective covering external to the spore-wall constitute two distinguishing features between seeds and megaspores. In Selaginella apus, a Californian species, Miss Lyon has shown that fertilisation of the egg-cell usually takes place while the megaspore is still in the strobilus. On examining withered decayed strobili of this species which had been partially covered with the soil for some months after fertilisation of the megaspores, several young plants were found with cotyledons and roots projecting through the crevices of the megasporangia[129]. From this, adds Miss Lyon, “it seems safe to assume that an embryo may have two periods of growth separated by one of quiescence quite comparable to those of seed plants with marked xerophilous features.”

In another Western American species S. rupestris described by the same writer the cotyledons of young plants were found protruding from the imbricate sporophylls of a withered cone (fig. 131, D). This species is interesting also from the occasional occurrence of one instead of four megasporangia in a sporangium; a condition which affords another connecting link between the heterosporous Pteridophytes, on the one hand, and the seed-bearing Phanerogams in which the occurrence of a single embryo-sac (megaspore) in each ovule is the rule. The cones of Selaginella rupestris retain connexion with the plant through the winter and fertilisation occurs in the following spring. After the embryo has been formed the megasporangium “becomes sunken in a shallow pit formed by the cushion-like outgrowth of the sporophyll around the pedicel.” It is 58suggested that this outgrowth may be comparable with the integument which grows up from the sporophyll in the fossil genus Lepidocarpon[130] and almost completely encloses the sporangium. In the drawings given by Miss Lyon no features are recognisable which afford a parallel to the integument of Lepidocarpon. I have, however, endeavoured to show, by a brief reference to this author’s interesting account of the two Californian species, that the physiological and morphological resemblances between the megasporangia of Selaginella and the integumented ovules of the seed-bearing plants are sufficiently close to enable us to recognise possible lines of advance towards the development of the true seed.

Professor Campbell[131] records an additional example of a Selaginella—probably S. Bigelovii—from the dry region of Southern California in which the spores become completely dried up after the embryo has attained some size, remaining in that state until the more favourable conditions succeeding the dry season induce renewed activity.

Isoetaceae.

The genus Isoetes is peculiar among Pteridophytes both in habit and in anatomical features. In its short and relatively thick tuberous stem, terminating in a crowded rosette of subulate leaves like those of Juncus and bearing numerous adventitious roots, Isoetes presents an appearance similar to that of many monocotyledonous plants. The habit of the genus is well represented by such species as Isoetes lacustris and I. echinospora[132] (fig. 132) both of which grow in freshwater lakes in Britain and in other north European countries. The latter species bears leaves reaching a length of 18 cm. The resemblance in habit between this isolated member of the Pteridophytes and certain Flowering plants, although in itself of no morphological significance, is consistent with the view expressed by Campbell that Isoetes may be directly related to the Monocotyledons[133].

59

Fig. 132. Isoetes echinospora (After Motelay and Vendryès).

1. Stem of I. lacustris.
2. Base of sporophyll: l, ligule; spg, sporangium partially covered by velum.

There is as a rule little or no difference between the foliage leaves and sporophylls; in I. lacustris the latter are rather60 larger and in the terrestrial species I. hystrix[134] the sterile leaves are represented by the expanded basal portions only, which persist like the leaf-bases of Lepidodendron as dark brown scales to form a protective investment to the older part of the stem. The innermost leaves are usually sterile; next to these are sporophylls bearing megasporangia, and on the outside are the older sporophylls with microsporangia. The long and slender portion of the leaf becomes suddenly expanded close to its attachment to the stem into a broad base of crescentic section which bears a fairly conspicuous ligule (figs. 132, B, l, 133, E, l) inserted by a foot or glossopodium in a pit near the upper part of the concave inner face. The ligule is usually larger than that of Selaginella, though of the same type. The free awl-like lamina contains four large canals bridged across at intervals by transverse diaphragms, and in the axial region a single vascular bundle of collateral structure. Other vascular elements, in the form of numerous short tracheids occur below the base of the transversely elongated ligule.

Stomata are found on the leaves of I. hystrix, I. Boryana[135], and in other species which are not permanently submerged. Both microsporangia and megasporangia are characterised by their large size and by the presence of trabeculae or strands of sterile tissue (fig. 133, E, H, t) completely bridging across the sporangial cavity or extending as irregular ingrowths among the spore-producing tissue. Similar sterile bands, though less abundant and smaller, are occasionally met with in the still larger sporangia of Lepidostrobus; these may be regarded as a further development of the prominent pad of cells which projects into the sporangial cavity in recent species of Lycopodium (fig. 126, D, p). The sporangia are attached by a very short stalk to the base of a large depression in the leaf-base below the ligule, from the pit of which they are separated by a ridge of tissue known as the saddle, and from this ridge a veil of tissue (the velum) extends as a roof over the sporangial chamber (fig. 133, E, v). In most species there is a large gap between the lower edge of the velum and that of the sporangial 61pit, but in I. hystrix this protective membrane is separated from the base of the leaf by a narrow opening, the resemblance of which to the micropyle of an ovule suggested to one of the older botanists the employment of the same term[136]. Mr T. G. Hill[137] has called attention to the presence of mucilage canals in the base of the sporophylls of I. hystrix, which he compares with the strands of tissue known as the parichnos accompanying the leaf-traces of Lepidodendron and Sigillaria in the outer cortex of the stem. The transverse section shown in fig. 133, H and I, shows two of these mucilage canals in an early stage of development; a strand of parenchymatous elements distinguished by their partially disorganised condition and more deeply stained membranes (fig. 133, I) runs through the spandrels of the sporophyll tissue close to the upper surface. There is a close resemblance between the structure of these partially formed mucilage-canals and the tissue which has been called the secretory zone in Lepidodendron stems. Fig. 133, H, also shows a large microsporangium with prominent trabeculae (t) lying below the velum. A longitudinal section (fig. 133, E) through a sporophyll-base presents an appearance comparable with that of an Araucarian cone-scale with its integumented ovule and micropyle. The megaspores are characterised by ridges, spines, and other surface-ornamentation[138]. Though usually unbranched, the perennial stem of Isoetes (fig. 132) has in rare cases been found to exhibit dichotomous branching, a feature, as Solms-Laubach[139] points out, consistent with a Lycopodiaceous affinity. The apex is situated at the base of a funnel-shaped depression. The stem is always grooved; in some species two and in others three deep furrows extend from the base up the sides of the short and thick axis towards the leaves: from the sides of these furrows numerous slender roots are given off in acropetal succession. A stele of peculiar structure occupies the centre of the stem; cylindrical in the upper part (fig. 133, A), it assumes a narrow elliptical or, in species in which there are three furrows, a triangular form in the lower portion of the tuberous stem.

The stem of I. lacustris represented in fig. 132, A, from which 63the laminae of the leaves have been removed from the summit affords an example of a species with two furrows. The drawing shows the widely gaping sides of the broad furrow with circular root-scars and a few simple and dichotomously branched roots. A short thick column of parenchymatous tissue projects from a slightly eccentric position on the base of the stem.

Fig. 133. Isoetes lacustris.

• A. Transverse section of stem: cr, cortex; x, x² xylem; c, cambium; a, thin-walled tissue; lt, leaf-traces; b, dead tissue.
• B, C, D. Portions of A enlarged.
• E. Longitudinal radial section of sporophyll-base: v, velum; l, ligule; bb; vascular bundle; m, megaspores; t, sterile tissue.
• F. Longitudinal section through the base of a root.
• G. Transverse section of root.
• H. Transverse section of sporophyll, showing sporangium with trabeculae, t; leaf-trace, (lt), and two groups of secretory cells.
• I. A group of secretory cells enlarged.

The primary vascular cylinder[140] consists of numerous spiral, annular or reticulate tracheids (fig. 133, A, x) which are either isodiametric or longer in a horizontal than in a vertical direction, associated with parenchyma. Lower in the stem crushed and disorganised xylem elements are scattered through a still living trabecular network of parenchymatous tissue. From the axial cylinder numerous leaf-traces (fig. 133, A, lt) radiate outwards, at first in a horizontal direction and then gradually ascending towards the leaves. The vascular cylinder is of the type known as cauline; that is, some of the xylem is distinct in origin from that which consists solely of the lower ends of leaf-traces. As in Lycopodium the development of the metaxylem is centripetal.

Von Mohl[141], and a few years later Hofmeister[142], were the first botanists to give a satisfactory account of the anatomy of Isoetes but it is only recently[143] that fresh light has been thrown upon the structural features of the genus the interest of which is enhanced by the many points of resemblance between the recent type and the Palaeozoic Lepidodendreae. A striking anatomical feature is the power of the stem to produce secondary vascular and non-vascular tissue; the genus is also characterised by the early appearance of secondary meristematic activity which renders it practically impossible to draw any distinct line between primary and secondary growth. A cylinder of thin-walled tissue (fig. 133, A, a) surrounds the primary central cylinder and in this a cambial zone, c, is recognised even close to the stem-apex; this zone of dividing cells is separated from the xylem by a few layers of rectangular cells to which the term prismatic zone has been applied. 64The early appearance of the cambial activity on the edge of the vascular cylinder is shown in fig. 133, C, which represents part of a transverse section of a young stem. A leaf-trace, lt, is in connexion with the primary xylem, x′, which consists of short tracheids, often represented only by their spiral or reticulately thickened bands of lignified wall, and scattered parenchyma. Some of the radially elongated cells on the sides of the leaf-trace are seen to be in continuity on the outer edge of the stele, at st, with flattened elements, some of which are sieve-tubes. The position of a second leaf-trace is shown at lt′. External to the sieve-tubes the tissue consists of radially arranged series of rectangular cells, some of which have already assumed the function of a cambium (c). The tissue produced by the cambium on its inner edge consists of a varying amount of secondary xylem composed of very short spiral tracheids; a few of these may be lignified (fig. 133, A, x²) while others remain thin.

Phloem elements, recognisable by the presence of a thickened reticulum enclosing small sieve-areas (fig 133, B, s) are fairly abundant, and for the rest this intracambial region is composed of thin-walled parenchyma. In longitudinal section these tissues present an appearance almost identical with that observed in a transverse section. Fig. 133, B represents a longitudinal section, through the intracambial zone and the edge of the stele, of a younger stem than that shown in fig. 133, A. Most of the radially disposed cells internal to the meristematic region are parenchymatous without any distinctive features; a few scattered sieve-tubes (s) are recognised by their elliptical sieve-areas and an occasional tracheid can be detected. The cambium cuts off externally a succession of segments which constitute additional cortical tissue (fig. 133, A, cr) of homogeneous structure, composed of parenchymatous cells containing starch and rich in intercellular spaces. As the stem grows in thickness the secondary cortex reaches a considerable breadth and the superficial layers are from time to time exfoliated as strips of dead and crushed tissue (fig. 133, A, b). The diagrammatic sketch reproduced in fig. 133, A, serves to illustrate the arrangement and relative size of the tissue-regions65 in an Isoetes stem. In the centre occur numerous spirally or reticulate tracheae scattered in parenchymatous tissue which has been considerably stretched and torn in the peripheral region of the stele; the radiating lines mark the position of the leaf-traces (lt) in the more horizontal part of their course. The zone between the cambium (c) and the edge of the central cylinder consists of radially disposed secondary tissue of short, and for the most part unlignified, elements including sieve-tubes and parenchyma; the secondary xylem elements consist largely of thin-walled rectangular cells with delicate spiral bands, but discontinuous rows of lignified tracheae (x²) occur in certain regions of the intracambial zone. The rest of the stem consists of secondary cortex (cr) with patches of dead tissue (b) still adhering to the irregularly furrowed surface. The structure of the cambium and its products is shown in the detailed drawing reproduced in fig. 133, D. Many of the elements cut off on the inner side of the cambium exhibit the characters of tracheids: most of these are unlignified, but others have thicker and lignified walls (tr).

I. hystrix appears to be exceptional in retaining its leaf-bases, which form a complete protective investment and prevent the exfoliation of dead cortex. Each leaf-trace consists of a few spiral tracheids accompanied by narrow phloem elements directly continuous with the secondary phloem of the intracambial zone. Dr Scott and Mr Hill have pointed out that a normal cambium is occasionally present in the stem of I. hystrix during the early stages of growth; this gives rise to xylem internally. The few phloem elements observed external to the cambium may be regarded as primary phloem, a tissue not usually represented in an Isoetes stem[144]. The occasional occurrence of this normal cambium, may, as Scott and Hill suggest, be a survival from a former condition in which the secondary thickening followed a less peculiar course. The lower leaf-traces become more or less obliterated as the result of the constant increase in thickness of the broad zone of secondary tissues through which they pass.

The adventitious roots are developed acropetally and 66arranged in parallel series on each side of the median line of the two or three furrows. The three arms of the triangular stele of I. hystrix and the two narrow ends of the long axis of the stele of I. lacustris, which in transverse section has the form of a flattened ellipse, are built up of successive root-bases. A root of Isoetes (fig. 133, G) possesses one vascular bundle, x, with a single strand of protoxylem, px, thus agreeing in its monarch structure with the root-bundle in Selaginella and many species of Lycopodium. The cortical region of the root consists of a few layers of outer cortex succeeded by a large space, formed by the breaking down of the inner cortical tissue, into which the vascular bundle projects (fig. 133, F). The peculiarity of the roots in having a hollow cortex and an eccentric vascular bundle was noticed by Von Mohl[145]. In the monarch bundles, as in the fistular cortex and dichotomous branching, the roots of Isoetes present a striking resemblance to the slender rootlets of the Palaeozoic Stigmaria (see page 246). The longitudinal section through the base of a root of Isoetes lacustris shown in fig. 133, F, affords a further illustration of certain features common to the fossil and recent types.

FOSSIL LYCOPODIALES.

Isoetaceae

The geological history of this division of the Pteridophyta is exceedingly meagre, a fact all the more regrettable as it is by no means improbable that in the surviving genus Isoetes we have an isolated type possibly of considerable antiquity and closely akin to such extinct genera as Pleuromeia and Sigillaria. If Saporta’s Lower Cretaceous species Isoetes Choffati[146], or more appropriately Isoetites Choffati, is correctly determined, it is the oldest fossil member of the family and indeed the most satisfactory among the more than doubtful species described as extinct forms of Isoetes.

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Isoetites.

The generic name Isoetites was first used by Münster[147] in the description of a specimen, from the Jurassic lithographic slates of Solenhofen in Bavaria, which he named Isoetites crociformis. The specific name was chosen to express a resemblance of the tuberous appearance of the lower part of the imperfectly preserved and indeterminable fossil to a Crocus corm.

Impressions of Isoetes-like leaves from the Inferior Oolite of Yorkshire figured by Phillips[148] and afterwards by Lindley[149] as Solenites Murrayana were compared by the latter author with Isoetes and Pilularia, but these leaves are now generally assigned to Heer’s gymnospermous genus Czekanowskia. An examination of the structure of the epidermal cells of these Jurassic impressions convinced me that they resemble recent coniferous needles more closely than the leaves of any Pteridophyte. The genus Czekanowskia[150] is recognised by several authors as a probable member of the Ginkgoales.

Isoetites Choffati. Saporta.

The late Marquis of Saporta founded this species on two sets of impressions from the Urgonian (Lower Cretaceous) of Portugal which, though not found in actual organic connexion, may possibly be portions of the same plant. Small relatively broad tuberous bodies reaching a breadth of 1 cm. are compared with the short and broad stem of Isoetes, which they resemble in bearing numerous appendages radiating from the surface like the roots of the recent species; on the exposed face of the stem occur scattered circular scars representing the position of roots which were detached before fossilisation. Other impressions are identified as the basal portions of sporophylls bearing sporangia: these suggest the expanded base of the fertile leaves of Isoetes with vertically elongated sporangia, some of which have a smooth surface while in others traces of internal structure are exposed; the interior consists of an irregular network with depressions containing carbonised remains of spores.

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While recognising a general resemblance to the sporophylls of Isoetes, certain differences are obvious: there is no ligule in the fossil leaves nor are there any distinct traces of vascular strands such as occur in the leaves of recent species. The form of the sporangium, more elongated than in the majority of recent forms, is compared by Saporta with that in a south European species Isoetes setacea Spr.

Such evidence as we have lends support to the inclusion of these Portuguese fossils in the genus Isoetites, but apart from the fact that we have no proof of any connexion between the stems and supposed sporophylls, the resemblance of the latter to those of Isoetes is, perhaps, hardly sufficient to satisfy all reasonable scepticism.

The generic name Isoetopsis was used by Saporta as more appropriate than Isoetes for some Eocene fossils from Aix-en-Provence which are too doubtful to rank as trustworthy evidence of the existence of the recent genus. The species, Isoetopsis subaphylla[151] is founded on impressions of small scales, 4 mm. long, bearing circular bodies which are compared with sporangia or spores.

Other records of fossils referred to Isoetes need not be described as they have no claim to be regarded as contributions towards the past history of the genus. Heer’s Miocene species Isoetites Scheuzeri and I. Braunii Unger[152] from Switzerland are based on unsatisfactory material and are of no importance.

Pleuromeia.

The generic name Pleuromeia, was suggested by Corda[153] for a fossil from the Bunter Sandstone, the original description of which was based by Münster[154] on a specimen discovered in a split stone from the tower of Magdeburg Cathedral.

The majority of the specimens have been obtained from the neighbourhood of Bernburg, but a few examples are recorded from Commern and other German localities: all are now included under the name Pleuromeia Sternbergi. Germar, who published 69one of the earlier accounts of the species, states that Corda dissented from Münster’s choice of the name Sigillaria and proposed the new generic title Pleuromeia. One of the best descriptions of the genus we owe to Solms-Laubach[155] whose paper contains references to earlier writers. Illustrations have been published by Münster, Germar[156], Bischof[157], Solms-Laubach and Potonié[158].

Pleuromeia Sternbergi. (Münster.)
Fig. 134.

Pleuromeia Sternbergi is represented by casts of vegetative and fertile axes, but the preservation of the latter is not sufficiently good to enable us to draw any very definite conclusions as to the nature of the reproductive organs. Casts of the stems reach a length of about 1 metre and a diameter of 5–6 cm., or in some cases 10 cm.; all of them are in a more or less decorticated state, the degree of decortication being responsible for differences in the external features which led Spieker[160] to adopt more than one specific name.

Fig. 134, A, represents a sketch, made some years ago, of a specimen in the Breslau Museum which contains several examples of this species, among others those described by Germar in 1852. The cylindrical cast (38 cm. long by 12 cm. in circumference), which has been slightly squeezed towards the upper end, bears spirally arranged imperfectly preserved leaf-scars and the lower end shows the truncated base of one of the short Stigmaria-like arms characteristic of the plant. As shown clearly in a specimen originally figured by Bischof and more recently by Potonié[161], the stem-base is divided by a double dichotomy into four short and broad lobes with blunt apices and bent upwards like the arms of a grappling iron (fig. 134, D). 70The surface of this basal region is characterised by numerous circular scars (fig. 134, D; 4 scars enlarged) in the form of slightly projecting areas with a depression in the centre of each. These are undoubtedly the scars of rootlets, remains of which are occasionally seen radiating through the surrounding71 rock. As seen in fig. 134, D, a, the fractured surface of a basal area may reveal the existence of an axial vascular cylinder giving off slender branches to the rootlets.

Fig. 134. Pleuromeia Sternbergi.

• A.  Cast of stem in the Breslau Museum (⅓ nat. size). (A.C.S.)
• B.  “Sigillaria oculina” Blanckenhorn. (After Weiss).
• C, D.  Leaf-scars and base of stem: a, vascular tissue. (After Solms-Laubach.)

The bulbous enlargement at the base of the Brown seaweed Laminaria bulbosa Lam.[162] simulates the swollen base of Pleuromeia; but a confusion between these two plants is hardly likely to occur. Above the Stigmaria-like base the gradually tapered axis, in the less decorticated specimens, bears spirally disposed transversely elongated areas consisting of two triangular scars between which is the point of exit of a leaf-trace. The form of the leaf-scars is best seen on the face of a mould figured by Solms-Laubach (fig. 134, C): in this case the two triangular areas appear as slight projections separated by a narrow groove marking the position of the vascular bundle of the leaf. The curved lines above and below the leaf-scar probably mark the boundary of the leaf-base. The two triangular scars are compared by Solms-Laubach and by Potonié with the parichnos-scars of Sigillaria and Lepidodendron (cf. fig. 146, C), but the large size of the Pleuromeia scars constitutes an obvious difference though possibly not a distinction of importance.

The occurrence of a vertical canal filled with carbonaceous material in some of the stems throws light on the internal structure: the canal, which is described by Solms-Laubach as having a stellate outline in transverse section recalls the narrow central cylinder of a Lepidodendron stem, and this comparison is strengthened by the presence of obliquely ascending grooves which represent leaf-traces passing through the cortex. In specimens which have lost more of the cortical tissues the surface is characterised by spirally disposed, discontinuous vertical grooves representing portions of leaf-traces precisely as they appear in similar casts of Lepidodendron. There is no direct evidence of the existence of secondary wood in the stem, but, as Potonié has pointed out, the greater transverse elongation of the leaf-scars in the lower part of a cast (fig. 134, A) points to the production of some secondary tissue either in the vascular cylinder or cortex, or possibly in both regions.

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In some specimens of Pleuromeia the upper portion is clothed with crowded and imbricate sporophylls which reach a length of 2·5 cm., a maximum breadth of 2·7 cm., and a thickness of 1 mm. Each sporophyll has a thin wing-like border, and on the lower face are several parallel lines. Solms-Laubach describes the sporangium or ovule as attached to the lower surface of the sporophyll and this opinion has been confirmed by Fitting[163] who has also brought forward satisfactory evidence in favour of the sporangial nature of the reproductive organs. Fitting found numerous spores in the Bunter Sandstone near Halle; these are flattened circular bodies 0·5–0·7 mm. in diameter with a granulated surface and the three converging lines characteristic of spores produced in tetrads. The comparison made by this author between the sporophylls of Pleuromeia, which bore the sporangia on the lower surface instead of on the upper as in other lycopodiaceous plants, and the pollen-sacs of Conifers, is worthy of note in reference to the possible relationship between Conifers and Lycopods.

A comparison of the Isoetes stem represented in fig. 132, A, with the base of a Pleuromeia shows a striking similarity, but, as Fitting points out, the Stigmaria-like arms of the fossil contained a vascular cylinder whereas the blunt lobes of Isoetes consist exclusively of cortical tissue, the roots being given off from the grooves between the lobes of the tuberous stem.

The position of Pleuromeia must for the present be left an open question; it is, however, clear that the plant bears a close resemblance in the form of its base to the Stigmarian branches of Lepidodendron and Sigillaria. The vegetative shoot appears to be constructed on a plan similar to that of these two Palaeozoic genera, but the strobilus is of a different type. It would seem probable that Pleuromeia may be closely allied to Isoetes and to the arborescent Lycopods of Palaeozoic floras. It is not improbably a link in a chain of types which includes Sigillaria on the one hand and Isoetes on the other.

It is not improbable that a specimen from the Lower Bunter of Commern which Blanckenhorn made the type of a new species, Sigillaria oculina (fig. 134, B) is specifically 73identical with Pleuromeia Sternbergi. An examination of a cast of the type-specimen in the Berlin Bergakademie led me to regard the fossil with some hesitation as a true Sigillaria, but a more extended knowledge of Pleuromeia lends support to the view adopted by Potonié[164] that Blanckenhorn’s plant is not genetically distinct from Pleuromeia Sternbergi. The resemblance between Sigillaria oculina and some of the Palaeozoic species of Sigillaria emphasised by Weiss[165] has given rise to the belief that the genus Sigillaria persisted into the Triassic era; it is, however, highly probable that the Bunter specimen has no claim to the generic name under which it has hither to been known.

The Bunter Sandstone in which Pleuromeia is the sole representative of plant-life, at least in certain localities, is usually considered to be a desert formation. We may not be far wrong in accepting Fitting’s suggestion that in this isolated species we have a relic of the sparse vegetation which was able to exist where the presence of lakes added a touch of life to the deadness of the Triassic desert.

Pleuromeia is recorded by Fliche as a rare fossil in the Middle Trias of France in the neighbourhood of Lunéville[166].

Herbaceous fossil species of Lycopodiales.

The history of our knowledge of fossil representatives of the Lycopodiales, as also of the Equisetales, affords a striking illustration of the danger of attempting to found a classification on such differences as are expressed by the terms herbaceous and arborescent in the sense in which they are usually employed. As we have seen[167], the presence of secondary wood in stems of the Palaeozoic plant now known as Calamites led so competent a botanist as Adolphe Brongniart to recognise a distinct generic type Calamodendron, which he placed in the Gymnosperms, reserving the designation Calamities for species in which no indication of secondary thickening had been found.

Similarly, the genus Sigillaria was regarded as a Gymnosperm because it was believed to be distinguished from 74Lepidodendron by the power of forming secondary vascular tissues; the latter genus, originally thought to be always herbaceous, was classed with the Pteridophytes. At the time when this unnatural separation was made between stems with secondary wood and those in which no secondary wood was known to exist, botanists were not aware of the occurrence of any recent Pteridophyte which shared with the higher plants the power of secondary growth in thickness provided by means of a meristematic zone. It is true that the presence or absence of a cambium does not in practice always coincide with the division into herbaceous and arborescent plants: no one would speak of a Date-Palm as a herbaceous plant despite the absence of secondary wood.

The danger which should be borne in mind, in adopting as a matter of convenience the term herbaceous as a sectional heading, is that it should not be taken to imply a complete inability of the so-called herbaceous types to make secondary additions to their conducting tissues. The specimens on which the species of Lycopodites and Selaginellites, (genera which may be designated herbaceous,) are founded are preserved as impressions and not as petrifications; we can, therefore, base definitions only on habit and on such features as are shown by fertile leaves and sporangia. We are fully justified in concluding from evidence adduced by Goldenberg more than fifty years ago and from similar evidence brought to light by more recent researches, that there existed in the Palaeozoic era lycopodiaceous species in close agreement in their herbaceous habit with the lycopods of present-day floras. It has been suggested[168] that the direct ancestors of the genera Lycopodium and Selaginella are represented by the species of Lycopodites and Selaginellites rather than by Lepidodendron and Sigillaria, the arborescent habit of which has been rendered familiar by the numerous attempts to furnish pictorial reproductions of a Palaeozoic forest. Until we are able to subject the species classed as herbaceous to microscopical examination we cannot make any positive statement as to the correctness of this view, 75but such facts as we possess lead us to regard the suggestion as resting on a sound basis.

Palaeobotanical literature abounds in records of species of Lycopodites, Lycopodium, Selaginella and Selaginites, which have been so named in the belief that their vegetative shoots bear a greater resemblance to those of recent lycopodiaceous plants than to the foliage shoots of Lepidodendron. Many of these records are valueless: Lepidodendra, twigs of Bothrodendron[169] species of conifers, fern rhizomes, and Aphlebiae[170] have masqueraded as herbaceous lycopods. It is obvious that an attempt to identify fossils presenting a general agreement in habit and leaf-form with recent species of lycopods must be attended with considerable risk of error. Recent Conifers include several species the smaller branches of which simulate the leafy shoots of certain species of Lycopodium and Selaginella, and it is not surprising to find that this similarity has been responsible for many false determinations. Among Mosses and the larger foliose Liverworts there are species which in the condition of imperfectly preserved impressions, might easily be mistaken for lycopodiaceous shoots: an equally close resemblance is apparent in the case of some flowering plants, such as New Zealand species of Veronica, Tafalla graveolens (a Composite), Lavoisiera lycopodiodes Gard.[171] (a species of Melastomaceae), all of which have the habit of Cupressineae among the conifers as well as of certain lycopodiaceous plants. It may be impossible to decide whether fossil impressions of branches, which are presumably lycopodiaceous, bear two kinds of leaves[172] like the great majority of recent species of Selaginella. Selaginella grandis, if seen from the under surface, would appear to have two rows of leaves only and might be confused with a small twig of such a conifer as Dacrydium Kirkii, a New Zealand species.

The New Zealand conifers Dacrydium cupressinum Soland. and Podocarpus dacrydioides Rich. closely simulate species of Selaginellites and Lycopodites: in the British Museum a 76specimen of the latter species bears a label describing it as Lycopodium arboreum (Sir Joseph Hooker and Dr Solander; 1769). The twigs of the Tasmanian conifer Microcachyrs tetragona Hook. f. are very similar in habit to shoots of the recent Lycopodium tetragonum (fig. 121, C).

In the description of examples of Lycopodites and Selaginellites I have confined myself to such as appear to be above suspicion either because of the presence of spore-bearing organs or, in a few cases, because the specimens of sterile shoots are sufficiently large to show the form of branching in addition to the texture of the leaves. The two generic names Lycopodites and Selaginellites are employed for fossil species which there are substantial grounds for regarding as representatives of Lycopodium and Selaginella. The designation Selaginellites is adopted only for species which afford evidence of heterospory; the name Lycopodites, on the other hand, is used in a comprehensive sense to include all forms—whether homophyllous or heterophyllous—which are not known to be heterosporous. This restricted use of the generic name Selaginellites is advocated by Zeiller[173], who instituted the genus, and by Halle[174] in his recent paper on herbaceous lycopods.

Lycopodites.

The generic term Lycopodites was used by Brongniart in 1822[175] in describing some Tertiary examples of slender axes clothed with small scale-like leaves which he named Lycopodites squamatus. These are fragments of coniferous shoots. In the Prodrome d’une histoire des végétaux fossiles[176] Brongniart included several Palaeozoic and Jurassic species in Lycopodites and instituted a new genus Selaginites, expressing a doubt as to the wisdom of attempting to draw a generic distinction between the two sets of species. In a later work[177] he recognised only one undoubted species, Lycopodites falcatus. The first satisfactory account of fossils referred to Lycopodites is by Goldenberg[178] 77who gave the following definition of the genus:—“Branches with leaves spirally disposed or in whorls. Sporangia in the axil of foliage leaves or borne in terminal strobili.”

It was suggested by Lesquereux[179] that Goldenberg’s definition, which was intended to apply to herbaceous species, should be extended so as to include forms with woody stems but which do not in all respects agree with Lepidodendron. Kidston[180] subsequently adopted Lesquereux’s modification of Goldenberg’s definition. We cannot draw any well-defined line between impressions of herbaceous forms and those of small arborescent species. We use the name Lycopodites for such plants as appear to agree in habit with recent species of Lycopodium and Selaginella and which, so far as we know, were not heterosporous: it is highly probable that some of the species so named had the power of producing secondary wood, a power possessed by some recent Pteridophytes which never attain the dimensions of arborescent plants.

It has been shown by Halle[181], who has re-examined several of Goldenberg’s specimens which have been acquired by the Stockholm Palaeobotanical Museum, that some of his species of Lycopodites are heterosporous and therefore referable to Zeiller’s genus Selaginellites.

In 1869 Renault described two species of supposed Palaeozoic Lycopods as Lycopodium punctatum and L. Renaultii[182], the latter name having been suggested by Brongniart to whom specimens were submitted. These species were afterwards recognised by their author as wrongly named and were transferred to the genus Heterangium[183], a determination which is probably correct; it is at least certain that the use of the name Lycopodium cannot be upheld.

We have unfortunately to rely on specimens without petrified tissues for our information in regard to the history of Lycopodites and Selaginellites. Among the older fossils referred to Lycopodites are specimens from Lower Carboniferous rocks at Shap in Westmoreland which Kidston originally described 78as Lycopodites Vanuxemi[184], identifying them with Goeppert’s Sigillaria Vanuxemi[185] founded on German material. In a later paper Kidston transferred the British specimens of vegetative shoots to a new genus Archaeosigillaria[186].

Lycopodites Stockii Kidston[187].

The plant so named was discovered in Lower Carboniferous strata of Eskdale, Dumfries, Scotland; it is represented by imperfectly preserved shoots bearing a terminal strobilus and was originally described by Kidston as apparently possessing two kinds of foliage leaves borne in whorls. The larger leaves have an ovate cordate lamina with an acuminate apex, while the smaller leaves, which are less distinct, are transversely elongated, and simulate sporangia in appearance. Dr Kidston’s figure of this species has recently been reproduced by Professor Bower[188] who speaks of the supposed smaller leaves as sporangia, a view with which the author of the species agrees. It would appear that this identification is, however, based solely on external resemblance and has not been confirmed by the discovery of any spores. Assuming the sporangial nature of these structures, this Palaeozoic type represents, as Bower points out, a condition similar to that in some recent species of Lycopodium in which sporangia are not confined to a terminal strobilus but occur also in association with ordinary foliage leaves. The strobilus consists of crowded sporophylls which are too imperfect to afford any definite evidence as to their homosporous or heterosporous nature. As Solms-Laubach[189] points out, this type recalls Lycopodium Phlegmaria among recent species.

Lycopodites Reidii Penhallow.

Professor Penhallow[190] instituted this name for a specimen measuring 8 cm. long by 6 mm. in breadth, collected by Mr 79Reid from the Old Red Sandstone of Caithness, consisting of an axis bearing narrow lanceolate leaves some of which bear sporangia at the base.

Lycopodites Gutbieri Goeppert[191].

1894, Lycopodites elongatus Kidston[192] (not Goldenberg).

The species, figured by Geinitz as Lycopodites Gutbieri[193], from the Coal-Measures of Saxony is probably a true representative of the genus. The Saxon specimens are heterophyllous; the larger lanceolate and slightly falcate leaves arranged in two rows, are 4–5 mm. long while the smaller leaves are one half or one third this size; some of the dichotomously branched shoots terminate in long and narrow strobili not unlike those of Zeiller’s species Selaginellites Suissei[194]. Kidston[195] has included under this specific name some fragments collected by Hemingway from the Upper Coal-Measures of Radstock, Somersetshire, but as only one form of leaf is seen the reasons for adopting Goeppert’s designation are perhaps hardly adequate.

Lycopodites ciliatus Kidston[196].

Under this name Kidston describes a small specimen, obtained by Hemingway from the Middle Coal-Measures of Barnsley in Yorkshire, consisting of a slender forked axis bearing oval-acuminate leaves approximately 5 mm. long with a finely ciliate margin. Associated with the leaves were found spores which Kidston regards as megaspores.

Lycopodites macrophyllus Goldenberg[197].

This species, originally described by Goldenberg from the Coal-Measures of Saarbrücken has been re-examined by Halle[198] who is unable to confirm Goldenberg’s statement as to heterophylly.80 The shoots closely resemble Selaginellites primaevus[199] (Gold).

Fig. 135. Selaginellites and Lycopodites. (After Halle.)

1. Selaginellites primaevus (Gold.). × 10.
2. Megaspore of Selaginellites elongatus (Gold.). × 50.
3. Lycopodites Zeilleri Halle. (Nat. size.)
4. Selaginellites elongatus (Gold.). × 2.

Lycopodites Zeilleri Halle[200]. Fig. 135, C.

Halle has founded this species on specimens, from the Coal-Measures of Zwickau in Saxony, characterised by dimorphic lanceolate leaves in four rows, the larger being 4–6 mm. long: the smaller leaves have a ciliate edge. A comparison is made with the recent species Selaginella arabica Baker, S. revoluta Bak., and S. armata Bak. in which the leaves are described as ciliate. In the absence of sporangia and spores the species is placed in the genus Lycopodites.

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Lycopodites lanceolatus (Brodie). Fig. 136.

Fig. 136. Lycopodites lanceolatus (Brodie). (After Miss Sollas. × 40.)
a, Sporangium wall; b, leaf.
c, remains of tubular elements in stem.

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Specimens referred to this species were originally recorded by Brodie from Rhaetic rocks in the Severn valley, the name Naiadita being chosen as the result of Lindley’s comparison of the small and delicate leaves with those of recent species of the Monocotyledonous family Naiadaceae. The species may be described as follows:

Plant slender and moss-like in habit. The axis, which is delicate and thread-like, bears numerous linear acuminate or narrow ovate leaves reaching a length of approximately 5 mm. Under a low magnifying power the thin lamina of the leaves is seen to have a superficial layer of polygonal or rectangular cells arranged in parallel series (fig. 136 b). There is no trace of midrib or stomata. The sporangia are more or less spherical and short-stalked, situated at the base of the foliage leaves and containing numerous tetrads of spores. The spores have a diameter of 0·08 mm.

Buckman founded additional species on differences in the shape of the leaves but, as Miss Sollas has pointed out, such differences as he noticed may be detected on the same axis. It was stated in an earlier chapter[208] that Starkie Gardner, on insufficient evidence, proposed to place Brodie’s plant among the Mosses. The discovery by Mr Wickes of new material at Pylle hill near Bristol afforded an opportunity for a re-examination of the species: this was successfully undertaken by Miss Sollas who was able to dissolve out spores from the matrix by dilute hydrochloric acid, and to recognise the remains of internal structure in the slender axes by exposing successive surfaces with the aid of a hone. It was found that sporangia occurred at the base of some of the leaves containing numerous tetrads of spores, the individual spores having a diameter of 0·08 mm., apparently twice as large as those of any recent species of Lycopodium. Fig. 136 shows a sporangium, a, at the base of a leaf, b. Indications of tubular elements were recognised in the stem and it is noteworthy that although the outlines of epidermal cells on the leaves are well preserved no stomata were found. The leaves of the recent American species Lycopodium alopecuroides Linn. var. aquaticum Spring[209], which lives 83under water, possess stomata. It is probable that in Lycopodites lanceolatus the leaves had a very thin lamina and may have been similar in structure to those of recent Mosses; the plant possibly lived in very humid situations or grew submerged. Miss Sollas’s investigations afford a satisfactory demonstration of the lycopodiaceous nature of this small Rhaetic species: as I have elsewhere suggested[210], the generic name Lycopodites should be substituted for that of Naiadita. Examples of this species may be seen in the British Museum.

The Rhaetic species from Scania, Lycopodites scanicus Nath.[211] (in litt.), recently re-described by Halle and originally referred by Nathorst to Gleichenia affords another example of the occurrence of a small herbaceous lycopod of Rhaetic age.

Fig. 137. Lycopodites falcatus L. and H. From the Inferior Oolite of Yorkshire. (Nat. size. M.S.)

Lycopodites falcatus Lind. and Hutt. Fig. 137.

In 1822 Young and Bird[215] figured a specimen from the 84Inferior Oolite rocks of the Yorkshire coast bearing “small round crowded leaves,” which was afterwards described by Lindley from additional material obtained from Cloughton near Scarborough as Lycopodites falcatus. The example represented in fig. 137 shows the dichotomously branched shoots bearing two rows of broadly falcate leaves. A careful examination of the type-specimen[216] revealed traces of what appeared to be smaller leaves, but there is no satisfactory proof of heterophylly. No sporangia or spores have been found. This British species has been recorded from Lower Jurassic or Rhaetic rocks of Bornholm[217] and a similar though probably not identical type, Lycopodites Victoriae[218], has been recognised in Jurassic strata of Australia (South Gippsland, Victoria). An Indian plant described by Oldham and Morris[219] from the Jurassic flora of the Rajmahal hills as Araucarites (?) gracilis and subsequently transferred by Feistmantel to Schimper’s genus Cheirolepis[220] may be identical with the Yorkshire species. The Jurassic fragments described by Heer from Siberia as Lycopodites tenerrimus[221] may be lycopodiaceous, but they are of no botanical interest.

Other examples of Mesozoic Lycopods have been recorded, but in the absence of well-preserved shoots and sporangia they are noteworthy only as pointing to a wide distribution of Lycopodites in Jurassic and Cretaceous floras[222].

From Tertiary strata species of supposed herbaceous lycopods have been figured by several authors, one of the best of which is Selaginella Berthoudi Lesq.[223] from Tertiary beds in Colorado. This species agrees very closely in the two forms of leaf with Selaginella grandis, but as the specimens are sterile we have not sufficient justification for the employment of the generic name Selaginellites.

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Selaginellites.

This generic name has been instituted by Zeiller[224] for specimens from the coal basis of Blanzy (France). It is applied to heterosporous species with the habit of Selaginella: Zeiller preferred the designation Selaginellites to Selaginella on the ground that the type species differs from recent forms in having more than four megaspores in each megasporangium. It is, however, convenient to extend the term to all heterosporous fossil species irrespective of the spore-output.

Selaginellites Suissei Zeiller.

This species was described in Zeiller’s preliminary note[225] as Lycopodites Suissei, but he afterwards transferred it to the genus Selaginellites. In habit the plant bears a close resemblance to Lycopodites macrophyllus of Goldenberg; the shoots, 1–3 mm. thick, are branched in a more or less dichotomous fashion and bear tetrastichous leaves. The larger leaves reach a length of 4–6 mm. and a breadth of 2–3 mm.; the smaller leaves are described as almost invisible, closely applied to the axis, oval-lanceolate and 1–2 mm. long with a breadth of 0·5–0·75 mm. Long and narrow strobili (15 cm. by 8–10 mm.) terminate the fertile branches; these bear crowded sporophylls with a triangular lamina and finely denticulate margin. Oval sporangia were found on the lower sporophylls containing 16–24 spherical megaspores 0·6–0·65 mm. in diameter. The outer membrane of the spore is characterised by fine anastomosing ridges and thin plates radiating from the apex and forming an equatorial collarette. The microspores have a diameter of 40–60μ and the same type of outer membrane as in the megaspores. The megaspores of the recent species Selaginella caulescens, as figured by Bennie and Kidston[226], resemble those of the Palaeozoic type in the presence of an equatorial flange. It is interesting to find that, in spite of the occurrence of 16–24 megaspores in a single sporangium the size of the fossil spores exceeds that of the recent species.

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Selaginellites primaevus (Gold.). Fig. 135, A, fig. 138.

Fig. 138. Selaginellites primaevus (Gold.). (After Goldenberg.)

In habit this species, first recorded by Goldenberg from the Coal-Measures of Saarbrücken, is similar to S. Suissei Zeill.

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The drawing reproduced in fig. 138 is a copy of that of the type-specimen: another specimen, named by Goldenberg, is figured by Halle in his recently published paper. The leaves appear to be distichous: no smaller leaves have been detected, though Halle is inclined to regard the plant as heterophyllous. The sporophylls, borne in slender terminal strobili, are smaller than the foliage leaves and spirally disposed (fig. 138; smaller specimen). Halle succeeded in demonstrating that some of the sporangia contained a single tetrad of spores, each spore having a diameter of 0·4–0·5 mm. No microspores were found, but it is clear that the species was heterosporous and that it agrees with recent species in having only four spores in the megasporangium.

Selaginellites elongatus (Gold.). Fig. 135, B, D.

The shoots of this species resemble the recent Lycopodium complanatum; they differ from those of Selaginellites primaevus in their long and narrow branches which bear two forms of leaf. The longer leaves, arranged in opposite pairs, are slightly falcate; the smaller leaves are appressed to the axis and have a triangular cordate lamina. Another peculiarity of this species is the occurrence of sporangia in the axil of the foliage leaves, a feature characteristic of the recent Lycopodium Selago. In recent species of Selaginella the sporophylls are always in strobili. No microspores have been found nor the walls of megasporangia, but tetrads of megaspores were isolated by Halle: the spores have three radiating ridges (fig. 135, B) connected by an equatorial ridge. Halle estimates the number of spores (0·45 mm. in diameter) in a sporangium at 20 to 30. In size as in number the spores exceed those of recent species and agree more nearly with the megaspores of S. Suissei.

It would seem to be a general rule that the spores (megaspores) of the fossil herbaceous species exceeded considerably in 88dimensions those of recent forms and on the other hand were smaller than those of the Palaeozoic arborescent species.

There can be little doubt that some of the Mesozoic and Tertiary species included under Lycopodites agree more closely with the recent genus Selaginella than with Lycopodium, but this does not constitute an argument of any importance against the restricted use of the designation Selaginellites which we have adopted. From a botanical point of view the various records of Lycopodites and Selaginellites have but a minor importance; they are not sufficiently numerous to throw any light on questions of distribution in former periods, nor is the preservation of the material such as to enable us to compare the fossil with recent types either as regards their anatomy or, except in a few cases, their sporangia and spores. The Palaeozoic species are interesting as revealing less reduction in the number of spores produced in the megasporangia. Among existing Pteridophytes the genus Isoetes agrees more closely than Selaginella, as regards the number of megaspores in each sporangium, with such fossils as Selaginellites Suissei and S. elongatus.

It would seem that in most Palaeozoic species heterospory had not reached the same stage of development as in the recent genus Selaginella in which the megaspores do not exceed four in each sporangium. In Selaginellites primaevus, however, the heterospory appears to be precisely of the same type as in existing species.

Lycostrobus.

The generic name Lycostrobus has recently been instituted by Nathorst[232] for certain specimens of a lycopodiaceous strobilus, from the Rhaetic strata of Scania, which he formerly referred to the genus Androstrobus[233].

Lycostrobus Scotti Nathorst. Fig. 139.

The fossil described under this name is of special interest as affording an example of a Mesozoic lycopodiaceous cone comparable in habit and in size with some of the largest examples 89of Palaeozoic Lepidostrobi, the cones of Lepidodendron. The Swedish fossil from Upper Rhaetic strata of Helsingborg (Scania) was originally designated Androstrobus Scotti, the generic name being adopted in view of the close resemblance of the form of the strobilus to the male flower of a Cycad. A more complete examination has shown that the bodies, which were thought to be pollen-sacs—though Nathorst recognised certain differences between them and the pollen-sacs of lycopods—are the megaspores of a lycopod. Microspores have also been identified. The axis of the cone has a breadth of 2 cm. with a peduncle which may be naked or provided with a few small scales; the90 sporophyll region of the axis reached a length of at least 12 cm. The spirally disposed sporophylls terminate in a rhombic distal end which may represent the original termination or they may have been prolonged upwards as free laminae. Each sporophyll bears on its upper face a single large sporangium containing either megaspores or microspores: the megaspores, 0·55–0·60 mm. in diameter, are finely granulate and bear small warty thorns or more slender pointed appendages. The microspores, after treatment with eau de Javelle, were found to measure 36–44μ while others which had been treated with ammonia reached 54μ in diameter. Nathorst describes the microspores as occurring in spherical groups or balls, which it is suggested may be compared with the groups of spores separated by strands of sterile tissue (trabeculae) in the large sporangia of Isoetes (cf. fig. 133, H). If this comparison is sound it would point to a more complete septation of the sporangium in Lycostrobus than in any recent species of Isoetes. The size of the strobilus would seem to indicate the persistence into the Rhaetic era of an arborescent lycopodiaceous type; but the appearance and manner of preservation of the axis is interpreted by Nathorst as evidence of a herbaceous rather than a woody structure. He is disposed to regard Isoetes as the most nearly allied existing genus.

Fig. 139. Lycostrobus Scotti, Nath. (After Nathorst; ⅘ nat. size.)

The comparison made by Nathorst with Isoetes is based on a resemblance between the spores of the two genera and on the evidence, which is not decisive, of the existence of sterile strands of tissue in the sporangia of Lycostrobus. This similarity is however hardly of sufficient importance to justify the inclusion of the Rhaetic strobilus in the Isoetaceae. In size and in the arrangement and form of the sporophylls the cone presents a much closer resemblance to Lepidodendron than to Isoetes. It is probably advisable to regard this Rhaetic type simply as a lycopodiaceous genus which we are unable, without additional information, to assign to a particular position.

The opinion expressed by Professor Fliche[234] that the plant described by Schimper and Mougeot as Caulopteris tessellata, a supposed tree-fern stem, from Triassic rocks of Lorraine, is 91more probably a large lycopodiaceous stem, either a Lepidodendron or a new genus, is worthy of note in reference to Nathorst’s account of Lycostrobus.

In habit the fossil strobilus may be compared with the Triassic genus Pleuromeia, but the position of the sporangia on the sporophylls constitutes a well-marked difference. The most important result of Nathorst’s skillful treatment of this interesting fossil by chemical microscopic methods is the demonstration of the existence of a large heterosporous type of lycopodiaceous cone in a Rhaetic flora.

Poecilitostachys.

Under this generic name M. Fliche[235] has briefly described a fertile lycopodiaceous shoot from the Triassic rocks of Epinal in France: the type species Poecilitostachys Hangi consists of a cylindrical axis, 10 cm. × 5 mm., deprived of leaves and terminating in a rounded receptacle bearing a capitulum of bracts or fertile leaves. Detached megasporangia containing small globular bodies found in association with the capitulum are compared with the megasporangia of Isoetes.

Among the best known plants in the Palaeozoic floras are the genera Lepidodendron and Sigillaria, types which are often spoken of as Giant Club-Mosses or as ancestors of existing species of Lycopodium and Selaginella. Of these genera, but more particularly of Lepidodendron, we possess abundant records in a condition which have made it possible to obtain fairly complete information not only in regard to habit and external features but as to the anatomical characters of both vegetative and reproductive shoots. The structure of Lepidodendron differs too widely from that of recent Club-Mosses (species of Lycopodium) to justify the statement that this prominent member of the Palaeozoic vegetation may be regarded as a direct ancestor of any living plant. There is at least no doubt that Lepidodendron and Sigillaria must be included in the Pteridophyta. The description by Dr Scott[236] of the genus Lepidocarpon, founded on petrified specimens of strobili, demonstrated the existence of a type of lycopodiaceous plant in the Carboniferous period distinguished from all living representatives of the group by the possession of integumented megaspores, which may fairly be styled seeds. Lepidocarpon and another seed-bearing plant Miadesmia are described under a separate heading as lycopodiaceous types characterised by an important morphological feature, which among recent plants constitutes a differentiating character between the Pteridophytes and the Phanerogams.

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Lepidodendron.

i. General.

The genus Lepidodendron included species comparable in size with existing forest trees. A tapered trunk rose vertically to a height of 100 feet or upwards from a dichotomously branched subterranean axis of which the spreading branches, clothed with numerous rootlets, grew in a horizontal direction probably in a swampy soil or possibly under water. A description by Mr Rodway[237] of Lycopods on the border of a savannah in Guiana forming a miniature forest of Pine-like Lycopodiums might, with the omission of the qualifying adjective, be applied with equal force to a grove of Lepidodendra. The equal dichotomy of many of the branches gave to the tree a habit in striking contrast to that of our modern forest trees, but, on the other hand, in close agreement with that of such recent species of Lycopodium as L. cernuum (fig. 123), L. obscurum (fig. 124) and other types. Linear or oval cones terminated some of the more slender branches (fig. 188) agreeing in size and form with the cones of the Spruce Fir and other conifers or with the male flowers of species of Araucaria, e.g. A. imbricata. Needle-like leaves, varying considerably in length in different species, covered the surface of young shoots in crowded spirals and their decurrent bases or leaf-cushions formed an encasing cylinder continuous with the outer cortex. The fact that leaves are usually found attached only to branches of comparatively small diameter would seem to show that Lepidodendron, though an evergreen, did not retain its foliage even for so long a period as do some recent conifers.

By the activity of a zone of growing tissue encircling the cylinder of wood the main trunk and branches grew in thickness year by year: the general uniformity in size of the secondary conducting elements affords no indication of changing seasons. As the branches grew stouter and shed their leaves the surface of the bark resembled in some degree that of a Spruce Fir and other species of Picea, in which the leaf-scars form the upper limit of 94prominent peg-like projections, which, at first contiguous and regular in contour, afterwards become less regular and separated by grooves (fig. 140) and at a later stage lose their outline as the bark is stretched to the tearing point (fig. 140, C). The leafless branches of Lepidodendron were covered with spirally disposed oval cushions less peg-like and larger than the decurrent leaf-bases of Picea, which show in the upper third of their length a clean-cut triangular area and swell out below into two prominent cheeks separated by a median groove and tapering with decreasing thickness to a pointed base, which in some forms (e.g. Lepidodendron Veltheimianum, fig. 185, C, D), is prolonged as a curved ridge to the summit of a lower leaf-cushion.

Fig. 140. Picea excelsa. Shoots of different ages showing changes in the appearance of the leaf-cushions: a leaf attached to a cushion in fig. A. (Slightly enlarged.)

A portion of the cushion below the triangular leaf-scar often shows transverse gaping cracks or depressions (fig. 185, C) such as occur on a smaller scale on the older cushions of a Fir twig (fig. 140). Secondary thickening, as in recent trees, is not confined to the vascular cylinder but at an early stage, frequently before there are any signs of secondary wood, the outer region of the broad cortex becomes the seat of active cell-formation which results in the addition of a considerable thickness to the bark. At a later stage of increase in girth, the leaf-cushions are stretched95 apart and the original surface-features become obliterated by vertical cracks and by the exfoliation of the superficial tissues[238].

Some species of Lepidodendron produced branches characterised by spiral or vertical series of scars; these in older shoots were replaced by depressions having a diameter of several inches and comparable in appearance, as also perhaps in manner of formation, with the scars left on the stem of a Kauri Pine (Agathis australis)[239] on the abscission of lateral branches by a natural process. These shoots, known as Ulodendron, are described in a subsequent section. (page 128.)

A fully-grown Lepidodendron must have been an impressive tree, probably of sombre colour, relieved by the encircling felt of green needles on the young pendulous twigs. The leaves of some species were similar to those of a fir while in others they resembled the filiform needles of the Himalayan Pine (Pinus longifolia). The occasional presence of delicate hyphae in the tissues of Lepidodendron demonstrates susceptibility to fungal pests.

Architecturally, if one may use the term, Lepidodendron owed its power of resistance to the bending force of the wind to its stout outer bark formed of thick-walled elements produced by the activity of a cylinder of cortical meristem (figs. 148, 172, etc.). The vascular axis, of insignificant diameter in proportion to the size of the stem (figs. 152, 153, 172, 181, A), must have played a subordinate part, from a mechanical point of view, as compared with the solid mass of wood of a Pine or an Oak.

Within the compass of a text-book it is impossible, even if it were desirable, to include an account of the majority of the species of the widely distributed Palaeozoic genus Lepidodendron. In spite of the great number of known species of this common member of Carboniferous floras, our knowledge of the type as a whole is deficient in many points, and such information as we possess needs systematising and extending by comparative treatment based on a re-examination of available data.

In order to appreciate the meaning of certain external 96features characteristic of Lepidodendron stems it is essential to have some knowledge of the internal structure.

A dual system of terminology has been unavoidably adopted for species of Lepidodendron: the majority of specific names have been assigned to fossils known only in the form of casts or impressions, while petrified fragments, which unfortunately seldom show the surface-features, have received another set of names. A glance at the older palaeobotanical literature reveals the existence of several generic designations, which fuller information has shown to have been applied to lepidodendroid shoots deprived of some of their superficial tissues before fossilisation and differing considerably in appearance from the more complete branches of the same species[240]. It has in some instances been possible to correlate the two sets of specimens, casts or impressions, showing external features, and petrified fragments. We may reasonably expect that future discoveries will enable us to piece together as definite specific types specimens at present labelled with different names.

A well-preserved leaf-cushion of a Lepidodendron—the most obvious distinguishing feature of the genus—is rhomboidal or fusiform and vertically elongated (fig. 146, C, E; fig. 185, C, D): in exceptional cases it may reach a length of 8 cm. and a breadth of 2 cm. The cushion as a whole represents a prominent portion of the stem or branch comparable with the elevation on the twig of a Spruce Fir and the leaf-base of a Lycopodium (cf. fig. 121, A, lower portion) which appears in a transverse section of a branch as a rounded prominence (cf. Lycopodium, fig. 125, A and H). Disregarding differences in detail, a typical Lepidodendron leaf-cushion is characterised by a clearly defined smooth area often situated in the middle region (fig. 146, C, s). This is the leaf-scar or place of attachment of the base of the leaf which was cut off by an absciss-layer while the branch was comparatively young, as in recent forest trees and in some species of Ferns. On the leaf-scar are three smaller scars or cicatricules, the central one is circular or more or less triangular in outline, the two lateral scars being usually oval or circular. The central pit marks the position of the single vascular bundle 97which constituted the conducting tissue connecting the leaf with the main vascular system of the stem. The two lateral scars (figs. 145, A, p; 146, C, s; 147, p) represent the exposed ends of two strands of tissue, the forked branches of a strand which pass from the middle cortex of the stem into the leaf; this is known as the parichnos, a name proposed by Professor Bertrand in 1891[241].

The specimen shown in fig. 141 shows the linear leaves attached to their respective cushions.

Fig. 141. Lepidodendron Sternbergii. From a specimen in the British Museum (No. v. 1235) from the Coal-Measures of Shropshire. (Nat. size.)

The lamina has a well-defined median keel on the lower surface and on either side a groove in which sections of petrified leaves have demonstrated the occurrence of stomata (cf. fig. 142).

ii. Leaves and Leaf-cushions.

All Lepidodendron leaves, so far as we know, possessed a single median vein only. In some species, as for example in Lepidodendron longifolium Brongn., they have the form of long 98and slender acicular needles very similar to those of Pinus longifolium; in L. Sternbergii (fig. 141) they are much broader and shorter. In external form as in internal structure it is often impossible to distinguish between the leaves of Lepidodendron and Sigillaria. The distinguishing features enumerated by the late M. Renault cannot be employed, with any great degree of confidence, as diagnostic characters. In transverse section the lamina of a Lepidodendron leaf presents the same appearance as that of the Sigillarian leaves represented in fig. 142. Near the base the free part of the leaf is usually sub-rhomboidal in section with short lateral wings, a ventral keel and two stomatal grooves (fig. 142, A, B, g). The form and arrangement of stomata are shown in fig. 143, A, which was drawn from a piece of a leaf shown in surface-view in a section lent to me by Professor Weiss. It should, however, be pointed out that the leaf cannot be certainly identified with Lepidodendron rather than with99 Sigillaria, but as the leaves of these two genera are constructed on the same plan the identification is of secondary importance.

Fig. 142. Leaves of Sigillaria in transverse section.

• A, A′. Section in the Manchester University Museum (Q. 631).
• B, C. Sections in Dr Kidston’s Collection.

The single xylem bundle consists of primary tracheae only, at least in such laminae as have been identified as Lepidodendroid. Surrounding the xylem strand occur delicate parenchymatous cells in some cases accompanied by darker and thicker-walled elements. As in Sigillaria, the leaves of which are more fully described on page 210, a fairly broad sheath of wider and shorter scalariform or spiral transfusion tracheids surrounds the conducting strand (figs. 142, t; 143, B, C, t). As Renault shows in the case of Lepidodendron esnostense[242], the small leaves of which are 1·5–2 mm. broad at the base and several centimetres long, the stomatal grooves and keel die out towards the apex when the lamina assumes a more nearly circular form (fig. 143, C).

Fig. 143.

• A. Stomata in surface-view (Lepidodendron?). a, parenchyma; t, transfusion tracheae; x, xylem. (Manchester University Collection R. 723).
• B, C. Lepidodendron esnostense Ren. (After Renault.)

The area of the cushion excluding the leaf-scar is spoken of by some writers as the field. Below the leaf-scar the kite-shaped cushion tapers to a gradually narrowing basal position: in Lepidodendron Veltheimianum, a species characteristic of 100Lower Carboniferous strata, it is seen to be continuous, as a ridge with sloping sides, with a lower cushion (fig. 185).

Below a leaf-scar the cushion frequently shows a pair of oval areas on which a fine pitting may be detected in well-preserved impressions, these oval scars, as seen in fig. 185, D, are practically continuous at the upper end with the parichnos scars on the leaf-scar area; this is explained by the fact that these infra-foliar scars also owe their existence to patches of lacunar, aerenchymatous tissue in close connexion with the parichnos[243].

Shortly before entering the base of the leaf-lamina the parichnos divides into two arms which diverge in the outer cortical region right and left of the vascular bundle, and passing obliquely upwards they come close to the surface of the leaf-cushion just below the leaf-scar. The diagram—fig. 144, B—shows a leaf-trace, lt, in the leaf-cushion, as seen in a diagrammatic drawing of a vertical radial section of a stem, the dotted lines, p, p′, show the two parichnos arms which are represented as impinging on the surface of the leaf-cushion at p′, and then bending upwards to pass into the leaf-base right and left of the vascular bundle or leaf-trace. For convenience the arms of the parichnos are represented in one plane though actually in different vertical planes.

Fig. 144, A, shows the difference between a view of the original surface of a Lepidodendron, as at a, where a leaf-cushion with a leaf-scar is seen, and a view of an impression representing the outer cortex, b, a short distance below the surface. The surface b, in fig. 144, A, corresponds to the face d-e in the diagrammatic longitudinal section fig. 144, B: the outline of each cushion is clearly visible and in the centre is seen the leaf-trace, lt, with its parichnos.

The surface-features, a (fig. 144, A), have been impressed on the rock, c, (fig. 144, B) in which the specimen was entombed and by the removal of the cast of the stem, that is the thickness b to e in fig. 144, B, the form of the leaf-cushion is revealed. The presence of the two infra-foliar parichnos scars at p′ (fig. 144, A) is explained by the diagram, fig. 144, B, p′.

The relation of the parichnos to the oval scars below a 101Lepidodendron leaf-cushion has been worked out in detail by Weiss who shows that, at least in some species, the two arms do not bend downwards as shown in the diagram, fig. 144, B, but pursue a straight gradually ascending course as seen in fig. 145, A. Just below the leaf-scar region of the cushion each arm comes into association with a group of lacunar, aerenchymatous tissue, such as occurs in the roots of certain Mangrove plants, and it is this aerenchyma which is exposed on the two oval depressions below the leaf-scar. The structure of this aerenchyma is shown in fig. 145, B; it consists in this species (L. Hickii Wats.) of stellate cells which would constitute an efficient aerating system. Probably, as Weiss suggests, these patches of aerenchyma were originally covered by an epidermis provided with stomata, and it is owing to the destruction of this superficial layer that the two oval scars often form a prominent102 feature on Lepidodendron leaf-bases[244]. The diagram reproduced in fig. 144, B, may be taken as practically correct, as the patches of aerenchyma described by Weiss do not differ essentially from the parichnos tissue.

Fig. 144. Lepidodendron Veltheimianum Sternb.

1. Leaf-cushion and leaf-scar seen in surface-view at a; on the rest of the specimen a slightly lower surface is exposed. (After Stur.)
2. Diagrammatic longitudinal section to explain the differences between its two surfaces a and b shown in fig. A.

   The shaded portion c represents the rock matrix, the surfaces ab, ed, mark the outer and inner edge of the outer portion of the bark of the Lepidodendron stem.

   lt, leaf-trace; p, p′, parichnos.

Fig. 145.

1. Diagrammatic surface-view and longitudinal section of a Lepidodendron leaf-cushion.
2. Aerenchyma below the leaf-scar. (After F. E. Weiss.)

The parichnos scars are shown on the leaf-scar and cushion in fig. 146, C. In the lower leaf-cushion shown in fig. 146, E, the infra-foliar parichnos scars, p, are clearly seen, but the preservation of the leaf-scar is not sufficiently good to show them on that part of the fossil. In the upper cushion (fig. 146, E) the position of the parichnos arms is shown on the leaf-scar, but the infra-foliar parichnos scars are hidden by two small spiral shells. The genus Spirorbis, to which these shells are referred, appears to have persisted from the Silurian epoch to the present day. The comparatively frequent occurrence of Spirorbis shells on the leaves and other parts of Palaeozoic plants, has recently 103been dealt with in a paper by Barrois[245] who discusses in detail the habitats of these small animals from the point of view of the conditions under which the plants were preserved. In a note by Malaquin appended to Barrois’ paper the belief is expressed that Spirorbis lived on pieces of Palaeozoic plants which lay under water.

The fact that with one exception all the Spirorbis shells on the specimen of Lepidodendron, of which two leaf-cushions are shown in fig. 146, E, occur on the large parichnos scars on the cheeks of the cushions, suggests the possibility that the escape of gases from the parichnos tissue may have rendered the position attractive to the Spirorbis. It can hardly be accidental that the shells occur on the parichnos strands. This fact recalls the view held by Binney[246] and accepted with favour by Darwin[247] that Lepidodendron and other coal-forest trees may have lived with the lower parts of the stems in sea water.

Above the leaf-scar is a fairly deep triangular or crescentic pit (fig. 146, C, l) known as the ligular pit from the occurrence on younger shoots of a delicate organ like the ligule of Isoetes (fig. 132) embedded in a depression in the upper part of the leaf-cushion. The ligule was first figured in Lepidodendron by Solms-Laubach[248] and described in English material by Williamson under the name of the adenoid organ[249].

In some Lepidodendron stems a second triangular depression may occur above the ligular pit, the meaning of which is not clear: this has been called the triangulum by Potonié[250]. Stur[251] suggested that it may represent the position occupied by a sporangium in Lepidodendron cones.

It is important to remember that as a branch increases in girth the leaf-cushions are capable of only a certain amount of growth: when the limit is reached they are stretched farther apart and thus the narrow groove which separates them is converted in older stems into a comparatively broad and flat channel, thus altering the surface characters.

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Fig. 146. Lepidophloios and Lepidodendron leaf-cushions.

• A, B, D, F, G, H, I.  Lepidophloios. (Fig. A should be reversed.)
• C, E.  Lepidodendron aculeatum.
• A, B.  From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 3 cm. broad).
• C.  From a specimen in the Sedgwick Museum, Cambridge (leaf-cushion 4 cm. long).
• D.  From a section in the Cambridge Botany School Collection.
• E.  From a specimen in the Bunbury Collection, Cambridge Botany School, showing Spirorbis shells (leaf-cushion 2 cm. long).
• F.  From a section in the Williamson Collection, British Museum No. 1, 973.
• G, H, I.  From sections in the Cambridge Botany School Collection.

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Another feature worthy of notice in reference to the leaf-cushions of Lepidodendron is the occurrence in rare instances of alternate zones of larger and smaller cushions. This variation in the size of the leaf-cushions is by no means uncommon in the closely allied genus Sigillaria; in Lepidodendron it has been described by Potonié[252] in L. volkmannianum and more recently by Mr Leslie and myself[253] in a South African species L. vereenigense.

Owing to the natural exfoliation of the superficial layers of the outer bark at a certain stage in the growth of the plant, or in some instances no doubt as the result of post-mortem decay, which destroys the delicate cells of the meristematic zone in the outer cortex, isolated leaf-cushions and strips of the external surface are occasionally met with as carbonised impressions.

The appearance presented by a Lepidodendron stem which has been deprived of its superficial tissues may be dealt with more intelligibly after we have become familiar with the anatomical characters.

iii. Lepidophloios.

Before proceeding further with the genus Lepidodendron a short account may be intercalated of the external features of a lepidodendroid type of stem which it is customary to describe under a distinct generic title Lepidophloios. This name is convenient for diagnostic purposes though it seems clear that apart from the form of the leaf-cushion (fig. 146, A) we are at present unable to recognise any well-defined differences between the two forms Lepidodendron and Lepidophloios. For general purposes the name Lepidodendron will be used as including plants possessing leaf-cushions of the type already described as well as those with the Lepidophloios form of cushion.

The generic name Lepidophloios was first used by Sternberg[254] for a Carboniferous species which he had previously described as Lepidodendron laricinum. In 1845 Corda[255] instituted the name Lomatophloios for specimens possessing the same external 106characters as those for which Sternberg had chosen the name Lepidophloios. The leaf-cushions of Lepidophloios differ from those of the true Lepidodendron in their relatively greater lateral extension (cf. fig. 146, A and C), in their imbricate arrangement and in bearing the leaf, or leaf-scar, at the summit. In some species referred to Lepidophloios the cushions are however vertically elongated and in this respect similar to those of Lepidodendron: an example of this type is afforded by Lepidophloios Dessorti a French species described by Zeiller[256]. In younger branches the cushions may be directed upwards having the leaf-scar at the top; but in the majority of specimens the cushions are deflexed as in figs. 146, D; 160, A. The shoot of Lycopodium dichotomum shown in fig. 121, B, with the leaves in the reversed position bears a close resemblance to a branch of Lepidophloios.

The photograph of Lepidophloios scoticus Kidst.[257] reproduced in fig. 160, A, illustrates the dichotomous branching of the stem and the form of the cushions with the leaf-scars pointing downwards. In the fertile branch of the same species shown in fig. 160, B, the leaf-scars face upwards.

In most species the cushions are simply convex without a median keel, but in some cases a median ridge divides the cushion into two cheeks as in the genus Lepidodendron. The leaf-scar bears three small scars, the larger median scar marking the position of the leaf-trace, while the lateral scars are formed by the two arms of the parichnos: in some examples of deflexed cushions, though not in all, a ligular pit occurs on the cushion a short distance above the leaf-scar.

The drawing reproduced in fig. 146, A, showing the leaf-scar on the upper edge of the cushion should have been reversed with the leaf-scars pointing downwards. This figure represents part of the surface of a specimen consisting of the outer cortex of a stem with leaf-cushions 3 cm. broad. The thickness of this specimen is 4 cm.: a section through the line ab is represented in fig. 146, D (reproduced in the correct position, with the leaf-scars, sc, pointing downwards): internal to the cushions is a band of secondary cortex (the shaded strip on the outer 107edge of the section) which was formed on the outside of the phellogen. The phellogen is a cylinder of actively dividing cells in the outer part of the cortex of the stem, often spoken of as the cork-cambium or cortical meristem, which produces a considerable amount of secondary cortical tissue on its inner face and a much smaller amount towards the stem surface. This delicate cylinder frequently forms a natural line of separation between the outer shell of bark and the rest of the stem. In the specimen before us, the thin-walled cells of the phellogen were ruptured before petrification and the outer shell of bark was thus separated as a hollow cylinder from the rest of the stem: this cylinder was then flattened, the two inner surfaces coming into contact. Fig. 146, D, represents a section of one half of the thickness of the flattened shell.

This separation of the outer cortex, and its preservation apart from the rest of the stem, is of frequent occurrence in fossil lycopodiaceous stems. The flattened outer cortical shell of a Lepidophloios, specifically identical with that shown in fig. 146, A and D, was erroneously described by Dr C. E. Weiss in 1881 as a large lepidodendroid cone[258].

Fig. 146, B, affords a view of the inner face of the specimen of which the outer surface is seen in fig. 146, A: the surface shown in the lower part of the drawing, on which the boundaries of the cushions are represented by a reticulum, corresponds to the inner edge of the strip of secondary cortical tissue represented by the vertically shaded band in fig. 146, D.

The shaded surface in fig. 146, B, represents a slightly deeper level in the stem which corresponds to the outer edge of the vertically shaded band of fig. 146, D: the narrow tapered ridges (fig. 146, B) represent the leaf-traces passing through the secondary cortex, and the fine vertical shading indicates the elongated elements of which this strip of secondary cortex is composed.

In the longitudinal section diagrammatically reproduced in fig. 146, D, cut along the line ab of fig. 146, A, the parenchymatous tissue of the stout cushions has been partially destroyed, as at a; at s is seen the section of a Stigmarian rootlet which has 108found its way into the interior of a cushion. Each leaf-trace is accompanied by a parichnos strand as in the true Lepidodendron; at the base of the leaf-cushion the parichnos branches into two arms which diverge slightly right and left of the leaf-trace, finally entering the base of the leaf lamina as two lateral strands (fig. 147, p). At one point in fig. 146, D the section has shaved a leaf-trace represented by a black patch resting on the parichnos just above the line ef, but it passes through one of the parichnos arms p′ which debouches on to the leaf-scar sc at p. Had the section been cut along the line cd of fig. 146, A the leaf-trace would have been seen in a position similar to that occupied by the parichnos p′ in fig. 146, D.

Fig. 147. Lepidophloios leaf-cushion in tangential section. (From a section in the Williamson Collection, British Museum, No. 1973.)

Fig. 147, A, affords a good example of a tangential section through a Lepidophloios leaf-cushion, 1 cm. broad, like that represented109 in fig. 146, A, showing the vascular bundle lt, the two parichnos strands, p, composed of large thin-walled cells (cf. Isoetes, fig. 133, H, I), and the ligular pit near the upper edge of the section enclosing the shrunken remains of the ligule (fig. 147, B, l).

Fig. 147, B, shows the form of the tangentially elongated leaf-cushions of Lepidophloios and their spiral disposition.

Fig. 146, F, represents a section similar to that shown in figs. 147, A and B, but in this case the leaf-trace, lt, and the parichnos strands, p, lie in a cavity formed by the destruction of some of the leaf-cushion tissue. It is worthy of notice that the parichnos cells have resisted decay more successfully than the adjacent tissue of the cushion.

The diagrammatic sketches reproduced in fig. 146, H and I, were made from a transverse section similar to one originally figured by Williamson[259]: fig. 146, H, corresponding in position to the line gh in fig. 146, A, passes through the ligular pit, l, and cuts across the parichnos in the act of branching; the leaf-trace passes outwards beyond the Y-shaped parichnos strand. In the other section, fig. 146, I, the parichnos is shown in a horizontal plane and the leaf-trace, lt, appears in oblique transverse section. In both sections and in fig. 146, G the shaded band at the base represents the secondary cortical tissue external to the phellogen.

The transverse section represented in fig. 146, G, shows in the left-hand cushion, a, the exit of the two parichnos arms and the leaf-trace between them: it illustrates also the various forms assumed by lepidodendroid leaf-cushions when cut across at different levels.

iv. The Anatomy of Lepidodendron vasculare Binney[260].
Figs. 148–155, 168, A.

In the earlier literature dealing with the anatomy of Lepidodendron and Sigillaria the presence or absence of secondary vascular tissue was made the criterion of generic distinction and the distinguishing feature between the classes Pteridophytes and 110Gymnosperms, Lepidodendron being relegated to the former class because it was supposed to have no power of forming secondary wood, while Sigillaria, characterised by a considerable development of such tissue, was classed by Brongniart and afterwards by Renault as a Gymnosperm. Binney[261] in 1865 recognised that the two types of stem pass into one another, but it was Williamson[262] who provided complete demonstration of the fallacy of the Brongniartian view.

These two undoubted Pteridophytes agree very closely in anatomical structure and both are now recognised as arborescent genera of Lycopodiaceous plants. In a paper published by Lomax and Weiss in 1905[263] a specimen is described from the Coal-Measures of Huddersfield, in which a decorticated stem with the anatomical characters of Binney’s Sigillaria vascularis gives off a branch having the anatomical structure which it has been customary to associate with the species Lepidodendron selaginoides, so-called by Sternberg and founded by him on impressions showing well-preserved external characters.

In 1862 Binney[264] described petrified specimens of vegetative shoots from the Lower Coal-Measures of Lancashire under the names Sigillaria vascularis and Lepidodendron vasculare. These were afterwards recognised as different states of the same species. A few years after the publication of Binney’s paper Carruthers[265] identified Binney’s species Lepidodendron vasculare with Sternberg’s L. selaginoides. The evidence on which this identification rests has not been stated, but many writers have retained this specific designation for the well-defined type of anatomical structure first described by Binney as L. vasculare. The use of the specific name selaginoides is, however, open to objection. The species Lepidodendron selaginoides, as pointed out by Kidston[266], is probably identical with the plant which Brongniart had named L. Sternbergii before the institution of Sternberg’s species, and we are not in possession of convincing evidence as to the connection of L. Sternbergii (= L. selaginoides) with specimens possessing the anatomy of Binney’s type. Binney’s designation is therefore retained for the anatomical type described in the following pages[267].

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The most detailed account hitherto published of the anatomy of Lepidodendron vasculare is that by the late M. Hovelacque[268], based on material from the Lower Coal-Measures of England.

Fig. 148. Lepidodendron vasculare Binney.

1. Transverse section. (Based on a section 2·5 cm. in diameter, in the Cambridge Botany School Collection.)
2. Longitudinal section. (Drawn from a section in Dr Kidston’s Collection.)

The small shoot, represented somewhat diagrammatically in fig. 148, A, illustrates the anatomical features of a typical example of the species: the shoot has a diameter of 2·5 cm. and its central cylinder (x-sc) is 2·5 mm. in width.

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Noticeable features are (i) the small size of the central cylinder (or stele) in proportion to the diameter of the branch, (ii) the production at a comparatively early stage of growth of a zone of secondary wood, x², which gradually assumes the form of a complete cylinder of unequal breadth, surrounding the primary xylem, x, (iii) the formation of a secondary cortical tissue by a meristematic cylinder (phellogen, pl) situated close to the leaf-cushion region of the outer cortex. On the outer edge the stele consists of narrow tracheae some of which show in longitudinal section the spiral form of thickening characteristic of most protoxylem elements: towards the centre of the stele the diameter of the tracheae gradually increases and parenchymatous cells become associated with the elongated scalariform elements. In the central region the stele is composed of parenchymatous tissue arranged in vertical series of short cells, interspersed with short tracheae distinguished by the greater thickness of their walls and by their scalariform and reticulate thickening bands. Some of these short tracheae are shown in vertical section in fig. 149, B: the fine and broken lines connecting adjacent thickening bands probably represent the remains of the original wall. These delicate bands, which have been figured in various species113 of lepidodendroid plants[269], are worthy of notice in connexion with the recent work of Mr Gwynne-Vaughan[270] who has shown that in many recent ferns the scalariform bands in the xylem elements are not connected by a thin pit-closing membrane, but are separated from one another by open spaces. In the Lepidodendron tracheae we seem to have a stage in which the intervening membrane is in process of absorption. It is, however, possible that the threads may be the result of contraction and splitting of the membrane during drying or decay.

Fig. 149. Lepidodendron vasculare. a, immature tracheae; m, meristem; mr, medullary ray; x, xylem.

1. Longitudinal section through the edge of the secondary wood.
2. Short tracheae in the centre of the stele. (From a specimen from the Halifax Hard bed in Dr Kidston’s Collection.)

The stele of Lepidodendron vasculare, before the addition of any secondary xylem, may be described as a protostele, a term originally proposed by Professor Jeffrey[271], in which the central part of the conducting strand of xylem elements has been converted into rows of parenchyma and short tracheids, the latter being better adapted to storage than to conduction. It is probable that this type of stelar anatomy, which distinguishes L. vasculare from other species, represents a comparatively primitive arrangement forming a transition between the stele of L. esnostense, which consists of a solid rod of tracheids, and the stele of L. Harcourtii (fig. 179, A) and other species in which the xylem forms a cylinder enclosing a large parenchymatous pith.

Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by an irregular reticulate pitting. The tangential section represented in fig. 148, B, illustrates the appearance of a shoot of L. vasculare in which no secondary xylem is present: the central strand of tissue consists of the parenchyma abutting on the xylem with several leaf-traces (lt) passing upwards in an almost vertical course from the outer edge of the stele.

The secondary xylem (fig. 148, A, x²) consists of radially arranged scalariform tracheae with associated rows of parenchymatous cells which form medullary rays (fig. 149, mr). Leaf-traces pass through the medullary rays in the secondary xylem cylinder in a direction at right angles to the primary xylem stele from which they are given off, but at the outer edge of the 114secondary xylem they bend suddenly upwards and for a time follow a steep and almost vertical course.

In well-preserved longitudinal sections the outermost secondary xylem tracheae are seen to be succeeded by a few narrow and vertically elongated elements (fig. 149, A, a), which represent young unlignified tracheae: these are followed by shorter parenchymatous cells (m) forming part of a meristematic zone from which the secondary xylem receives additions.

Returning to fig. 148, A; the zone of secondary wood, x², composed of scalariform tracheids and medullary rays, is succeeded by a few layers of parenchymatous cells and beyond this is a broader zone, sc, to which the term secretory zone has been applied[272]; this is made up of small parenchymatous cells varying in size and of larger spaces which appear to have been formed by the disorganisation of thin-walled elements. The whole zone presents a characteristic appearance due to the association of small cells, large clear spaces, and a certain amount of dark-coloured material suggestive of tissue disorganisation and secreted products. The anatomical characters of the secretory zone are shown in the photograph, fig. 168, A, sc. Several leaf-traces are seen in transverse section in the secretory zone (black dots in fig. 148, A, sc; fig. 154, C, lt): each trace consists of a strand of narrow tracheae accompanied by a few encircling layers of small parenchymatous cells. As a trace continues its steeply ascending course through the secretory zone, it becomes associated with a strand of that tissue and assumes the form of a collateral vascular bundle, the outer part of which does not consist of typical phloem but of shorter elements derived from the secretory zone. Beyond the secretory zone we find a more homogeneous tissue composed of parenchymatous elements slightly extended tangentially (figs. 148, A, c¹; fig. 168, A, c); this is spoken of as the inner cortical region. In the great majority of sections of L. vasculare as of other species of the genus, the broader middle cortex (fig. 148, c²) is occupied by mineral matter, introduced subsequent to decay of the tissue; or it is represented by patches of delicate tissue composed of loosely arranged parenchymatous cells varying considerably in 115size and shape, some being small, oval or polygonal elements while others have the form of sinuous hypha-like tubes.

In this middle cortical region may be seen leaf-traces passing outwards in an almost horizontal course (fig. 148, A, lt): after leaving the inner cortex the leaf-traces bend somewhat abruptly outwards to follow a more direct path through the middle and outer cortex. The ring of tissue, s, seen in the middle cortex of fig. 148, A, belongs to a Stigmarian rootlet.

The outer cortex (fig. 148, A and B, c³) consists of homogeneous parenchyma which is stronger and more resistant to decay than the looser middle cortex. The leaf-traces, as shown in fig. 148, B, pass through this region in a rather steeply ascending direction: each is seen to be enclosed by a space originally occupied by a strand of middle cortical tissue which accompanies lepidodendroid leaf-traces on their under side and has already been described as the parichnos, (pp. 97, 100–103; figs. 146, 147).

The surface of the stem shown in section in fig. 148, A, is composed of broad leaf-cushions. A single leaf-trace with its parichnos passes into each cushion, but in the neighbourhood of the base of a cushion the parichnos bifurcates (cf. fig. 146, H, I) and the arms diverge slightly to the right and left finally passing beyond the cushion into the lamina of the leaf, their position being shown, as already explained, by the two small lateral scars on the leaf-scar area.

The diagrammatic sketch of a radial longitudinal section through a leaf-cushion represented in fig. 150 illustrates the relation of the leaf-trace to the leaf-cushion. The trace consists of xylem, x, above and a strand of the secretory zone, st, below; the parichnos tissue was originally present on the under side of the leaf-trace at a. The external surface, bc, marks the limit of the leaf-scar through the middle of which passes the vascular strand lt.

The lower gap a has been formed by the tearing of thin-walled cells of the phellogen, the meristematic tissue from which a considerable amount of secondary cortical tissue or phelloderm has been produced at pd. On the outside of the cushion, c, the cells are somewhat crushed and distinguished116 by their darker colour from the bulk of the parenchymatous tissue d.

This section also illustrates another characteristic feature of Lepidodendron, namely the presence of a ligule and a ligular pit: the former is represented by a carbonised patch of tissue and the latter extends from the surface of the cushion at b, just above the leaf-scar, almost to the level of the leaf-trace, lt. A comparison of this section with figs. 146 and 147 will make clear the relation of the several parts of the cushion and leaf-scar.

The gaps gg, seen in fig. 148, A and B, mark the position of the delicate meristematic zone or phellogen which arises close to the bases of the leaf-cushions; the phellogen has already produced a few rows of radially disposed elements, represented by short radial lines in the drawing, which constitute secondary cortical tissue.

Fig. 150. Lepidodendron vasculare. Leaf-cushions in longitudinal section. (From a specimen in Dr Kidston’s Collection.)

In older shoots the amount of the secondary cortical tissue developed on the inner side of the phellogen is considerable (cf. figs. 152, 153).

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The structure of the cortex of a shoot in which secondary growth, both in the stele and in the outer cortex, has progressed further than in the specimen shown in fig. 148 is represented in fig. 151.

Fig. 151. Lepidodendron vasculare. An older stem than that shown in fig. 148. (From a section in the Manchester Museum. No. 351.)

The section (fig. 151, A) measures 7 × 3·8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood (fig. 151, E, x and x²). The almost smooth surface of the primary wood (fig. 151, E, x) is succeeded by the secondary xylem, x², characterised at its inner edge by the tapered ends of the radial rows of scalariform tracheids between which occur several delicate parenchymatous cells (fig. 151, E, a). The occurrence of such isodiametric elements, often exhibiting a delicate spiral thickening band, is a characteristic feature of the boundary between primary and secondary wood in lepidodendroid stems. The secondary wood is penetrated by numerous medullary rays and in some of them are seen strands of narrow spirally thickened tracheae—the leaf-traces—which are in organic continuity with the exarch protoxylem of the primary wood. The leaf-traces are oval and mesarch. The space, c², (fig. 151, A) originally occupied by the delicate middle cortex, is succeeded by a shell of outer cortex composed chiefly of118 secondary tissue (phelloderm, pd) passing towards the inner boundary of this region into the primary outer cortex g (fig. 151, A and C). The radially disposed elements which make up the bulk of the phelloderm are associated with concentric rows of secretory strands, represented by tangentially arranged dots in fig. 151, A: on the outer edge of the phelloderm a few patches of primary cortex are still preserved, as at c, fig. A. One of these is shown on a larger scale in fig. B; at m the phelloderm is interrupted by a gap beyond which the cells have thinner walls and show signs of recent division; this is probably the position of the phellogen. The tissue b, fig. 151, B, consists of secondary cortex succeeded beyond d by the parenchymatous tissue of the leaf-cushion, in which the remains of a ligule, l, are seen in the ligular pit. This section corresponds in position to a line drawn across fig. 150 at the level of b. In this specimen we have two kinds of secondary cortical tissue: that formed external to the phellogen, from m to d in fig. 151, B, is less in amount than that produced internal to the phellogen. We cannot make any satisfactory statement as to the nature of this secondary tissue, whether or not any of it agreed in composition with the cork which is usually formed external to the phellogen in recent plants. As the stem of a Lepidodendron grew in girth the leaf-cushions became separated by intervening depressions composed of the secondary cortex formed external to the phellogen, but at a later stage the cushions were thrown off, leaving the outer edge of the phelloderm as the superficial tissue. This exposed tissue became fissured as growth and consequent stretching continued, producing the appearance seen on the surface of the still older stem represented in fig. 153.

The inner edge of the phelloderm seen at e in fig. 151, C, passes suddenly into the inner primary region of the outer cortex (fig. 151, A and C, g) which comprises two types of parenchymatous tissue, patches of isodiametric cells, g, g, alternating with radially arranged areas consisting of tangentially elongated elements (fig. C, f, f; fig. D) which extend as wedges into the phelloderm.

The longitudinal section represented in fig. 152, B, shows an equal bifurcation of a stem in which no secondary xylem is119 present; in the lower part of the section the xylem and the outgoing leaf-traces are seen in radial section and at the upper end of each arm the leaf-traces alone, lt, are exposed, as in fig. 148, B. It is interesting to notice the large amount of phelloderm which has been produced in the fork of the branch, at pd, where greater strength is required.

Fig. 152. Lepidodendron vasculare. Sections of dichotomously branched shoot.

1. From a section (10·5 × 9 cm.) in the Cambridge Botany School Collection.
2. From a section (8 cm. long) in the Cambridge Collection.

The section represented diagrammatically in fig. 152, A, has lost the outermost part of the cortex together with the leaf-cushions; 120it consists largely of secondary cortex composed of radially disposed phelloderm cells and tangentially placed secretory strands (represented by the discontinuous black lines in the drawing): the dotted region in the central part of the axis is composed of primary cortical parenchyma, and the two spaces surrounding the steles contain portions of the lacunar middle cortex. Each stele possesses a narrow crescentic zone of secondary xylem; the amount is greater in the case of the right-hand stele, of which a small piece is shown on a larger scale; the striking contrast in size between the outer and more internal secondary tracheae is no doubt the expression of some unfavourable condition of growth. The position of the secretory zone beyond the secondary xylem is shown at sc, fig. 152, A.

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Fig. 153. Lepidodendron vasculare.
(From a specimen (16 × 7·5 cm.) in the Manchester Museum.)

An example of a large and partially decorticated stem is afforded by the specimen (16 × 7·5 cm.) shown in fig. 153. The irregularly ribbed surface is formed of rather thick-walled phelloderm, in which occur tangentially arranged rows of secretory strands. The tapered form of the secondary cortex as it abuts internally on the primary cortex is shown very clearly in the drawing (cf. fig. 151, C). The stele in this much older stem consists mainly of secondary wood.

Fig. 154. Lepidodendron vasculare. Shoot (2·8 cm. diam.) with two steles. (From a specimen from Halifax in the Williamson Collection, British Museum, No. 340.)

An interesting example of a small shoot, the largest diameter of which is 2·8 cm., is shown in fig. 154, A: the section was cut a short distance above the bifurcation of the stele into two approximately equal branches. The outer part of the cortex consists of phelloderm, pd, with the usual rows of secretory tracts, and primary outer cortex g; the middle cortex is represented by patches of parenchyma with a few leaf-traces. To one of the steles, s′ (fig. 154, A), a crescent-shaped band of122 secondary xylem has been added; the other stele, S, possesses no fully developed secondary elements.

Fig. 154, B and C, illustrates the anatomical features immediately external to the primary xylem of the smaller stele, s. The comparatively broad band of radially disposed parenchyma, m, is connected with the outermost elements of the xylem by a few rather dark and small crushed parenchymatous cells. The band m, which we may speak of as the meristematic zone, clearly consists of cells in a state of division; it is in this region that the secondary xylem is produced. Beyond the leaf-trace, (fig. 154, C, lt), occurs a portion of the secretory zone, some of the smaller cells of which show signs of disorganisation; but most of this tissue has been destroyed (fig. 154, B, sc). The outer edge of the secretory zone is shown in fig. 154, D abutting on the cells of the inner cortex, c′. The leaf-trace shown in the inner cortex in fig. 154, B illustrates the more oval or tangentially extended form of the xylem in this region, in contrast to the more circular outline which it exhibits on the inner side of the secretory zone.

Fig. 155. Lepidodendron vasculare. Outer edge of secondary xylem: m, meristematic zone; mr, medullary ray. (Drawn from the section shown in fig. 168, A).

The transverse section, part of which is reproduced in fig. 168, A, illustrates a characteristic feature, namely the juxtaposition of the outermost tracheae of the secondary xylem123 and much smaller cells of the meristematic zone. This is seen in fig. 155, which shows a small piece of fig. 168, A, on a larger scale. In plants with a normal cambium the segments cut off from the initial layer fit on to the elements of the xylem or phloem to which they are to form additions, but in Lepidodendron it seems to be a general rule to find each of the most external lignified elements abutting on a group of two or three much smaller cells. It is difficult to believe that the meristem shown in fig. 155, m, could produce secondary xylem elements equal in size to those already formed: in all probability had growth continued there would have been a marked difference between the size of the secondary tracheids, as in fig. 152, A, x², where there was no doubt some cause which interfered with normal cambial activity. This disparity in size between the secondary xylem elements and the adjacent parenchymatous tissue of the meristematic zone is by no means exceptional and may be described as the general rule. It is at least certain that in Lepidodendron vasculare, as in other species, the secondary xylem was succeeded by a broad band of parenchymatous tissue, from which new tracheae and medullary-ray elements were produced, and not by a narrow cambium such as occurs in recent plants.

v. Lepidodendron stems as represented by casts and impressions of partially decorticated specimens.

The differentiation of the outer cortex of a Lepidodendron into comparatively thin-walled and more resistant tissue has been the cause of unequal decay and the consequent formation of shrinkage cavities. In addition to the unequal resisting power of contiguous tissues, another important factor in determining the nature of casts and impressions is the existence of the cylinder of delicate cells in the outer cortex of stems and branches. As already pointed out, this meristematic cylinder or phellogen constitutes a natural line of separation, as in the case of the cambium layer between the wood and the external tissues in a fresh Sycamore twig. The result of the separation of an outer shell of bark from the rest of the stem and the124 results of unequal decay in the more superficial tissues, have necessarily led to the preservation of the same specific type under a variety of forms.

Our knowledge of the anatomy of Lepidodendron stems enables us to recognise in fossils of very different appearance specimens in various conditions of preservation of one and the same type. Such names as Knorria, Bergeria and Aspidiaria are examples of generic titles instituted before any adequate knowledge of Lepidodendron anatomy was available.

Differences in age as well as different degrees of decortication have contributed in no small measure to the institution of generic and specific names which more recently acquired knowledge has shown to be superfluous.

a. Knorria.

The designation Knorria, after a certain G. W. Knorr of Nürnberg, was proposed by Sternberg in 1826[273] for casts of Palaeozoic stems of a type figured more than a century earlier by Volkmann[274]. Goeppert, in his earlier works, published drawings of fossil stems which he referred to Sternberg’s genus: one species he at first called Didymophyllum Schollini. He afterwards[275] described some specimens which showed that the features characteristic of Knorria may occur on partially decorticated stems with leaf-cushions of the true Lepidodendron type. His specimens, preserved in the Breslau Museum, demonstrate the accuracy of his drawings and conclusions. Goeppert, and after him Balfour[276], drew attention to the different appearances presented by branches of Araucaria imbricata when preserved with the surface intact and after partial decortication, as illustrating possible sources of error in the determination of fossil stems.

Although it is now a well-established fact that fossils bearing the name Knorria are imperfect lepidodendroid stems, the use of the term may be conveniently retained for descriptive purposes. The specimen from the Commentry coal-field of 125France, shown in fig. 156, affords some excuse for the institution of several generic names for different states of preservation or decortication of one species. The cortical level exposed at e is characterised by spirally disposed peg-like ridges with truncated apices: it is this form of cast which is usually designated Knorria. The ridges vary in size and shape in different types of stem; they may be narrow as shown at e, fig. 156, or short and broad with rounded distal ends. In some cases they are forked at the apex, as in the partially decorticated specimen of Lepidodendron Veltheimianum represented in fig. 185, A.

Fig. 156. A dichotomously branched Lepidodendroid stem (Knorria mirabilis Ren. and Zeill.). (After Renault and Zeiller.) (¼ nat. size.) The original specimen is in the Natural History Museum, Paris.
a–g, surface features exposed as the result of different degrees of decortication. (See vol. I. p. 102, fig. 23).

The Knorria state represents the impression or cast of the outer cortical region too deep below the leaf-cushion region to126 retain any indications of the cushion-form; the ridges are the casts of the spaces produced in the cortex by the decay of the sheath of delicate cells surrounding each leaf-trace and by the decay of the thin-walled cells of the parichnos. The occasional forked apex of a ridge is the expression of the fact that the cast was made at the region where the parichnos divides into two arms (cf. p. 100). In certain specimens it is possible to connect the Knorria casts with associated lepidodendroid stems which may be determined specifically; but when we have no evidence as to surface-features the fossils may be designated casts of lepidodendroid stems in the Knorria condition. Such casts are illustrated by numerous drawings in palaeobotanical literature[277].

b. Bergeria.

This is another name first used by Sternberg in his classic work, Die Flora der Vorwelt, for casts of lepidodendroid plants such as Steinhauer[278] had previously figured as Phytolithus cancellatus. Brongniart[279] recognised that the application of the generic title Lepidodendron should be extended to include specimens referred by Sternberg to Bergeria, and a few years later Goldenberg[280] realised that this name does not stand for well-defined generic characters. The correctness of these views was, however, first satisfactorily demonstrated by Carruthers[281] and by Feistmantel[282].

If a Lepidodendron stem loses its superficial layers of outer cortex and in this condition is embedded in sand or mud, the cast is distinguished from that of a perfect stem by the absence of the leaf-scars and by other features. It may, however, still show spirally disposed areas, corresponding approximately to the original leaf-cushions, which are characterised by a small depression or pit either at the apex or near the centre of each oval area: the pit marks the position of the leaf-trace and its parichnos strand. In some cases the exposed surface may be smooth without any indication of leaf-cushions, while narrow 127spirally arranged grooves represent the obliquely ascending vascular bundles passing through the cortex to the leaves.

Fig. 185, B, shows the Bergeria state of Lepidodendron Veltheimianum, which differs from the Knorria condition in the fact that decortication had not extended below the level at which the form of the leaf-cushions could be recognised. It is clear that no sharp line can be drawn in all cases between the different degrees of decortication as expressed by the terms Knorria and Bergeria.

A list of synonyms of Knorria, Bergeria, and Aspidiaria forms of stem and a detailed treatment of their characteristic features may be found in a recent work by Potonié[283].

c. Aspidiaria.

In one of the earliest English books on fossil plants, the Antediluvian Phytology by Artis[284], a specimen from the Carboniferous sandstone of Yorkshire is figured as Aphyllum cristatum, and a similar fossil is described as A. asperum. These are impressions of Lepidodendron stems in which the characteristic leaf-cushions are replaced by smooth and slightly convex areas with a narrow central ridge. To this type of specimen Presl gave the name Aspidiaria[285], under the impression, shared by subsequent writers, that the supposed external features were entitled to generic recognition.

It is to Stur[286] that we owe the first satisfactory interpretation of fossils included under the name Aspidiaria: he showed that on the removal of the projecting convex areas from some of his specimens a typical Lepidodendron leaf-cushion was exposed (fig. 144, A, a). The Aspidiaria condition (fig. 144, A, b) represents the inner face of the detached shell of outer bark of a Lepidodendron stem, while in the Bergeria casts we have a view of the external face of a stem deprived of its superficial tissues.

In a Lepidodendron stem embedded in sediment the more delicate portions of the leaf-cushions would tend to shrink away 128from the internal and more resistant tissues of the outer cortex, thus producing spaces between each cushion; further decay would cause rupture of the leaf-traces and the superficial tissues would thus be separated from the rest of the stem. The tendency of Lepidodendron stems to split along the line of phellogen in the outer cortex is seen in fig. 148, A, g. The deposition of sediment on the exposed inner face of this cortical shell would result in the production of a specimen of the Aspidiaria type: the reticulum enclosing the spirally disposed convex areas is formed by the impression of the firmer tissue between the leaf-cushions.

vi. Lepidodendroid axes known as Ulodendron and Halonia.

a. Ulodendron.

This generic name was suggested by Lindley and Hutton[287] for two specimens from the English Coal-measures characterised by leaf-cushions like those of a Lepidodendron, but distinguished by the presence of two vertical rows of large and more or less circular cup-shaped scars. These authors, while recognising the possibility that the fossils might be identical with Lepidodendron, regarded them as generically distinct. The generic title Ulodendron, though no longer denoting generic rank, is still applied to certain shoots of lycopodiaceous plants which may belong to the genera Lepidodendron, Bothrodendron, and according to some authors[288], also to Sigillaria.

The large specimen from the Belgian coal-measures, represented in fig. 211, affords a good example of the Ulodendron form of shoot of the genus Bothrodendron, which is described on page 249. The specimen shown in fig. 157 shows the Ulodendron shoot of Lepidodendron Veltheimianum.

Casts of large Ulodendron scars are occasionally met with as separate fossils bearing a resemblance to an oval shell.

In Steinhauer’s paper on Fossil Reliquiae[289] a drawing is given 129of a Ulodendron stem under the name Phytolithus parmatus and a similar stem specifically identical with that shown in fig. 157 was figured by Rhode[290], one of the earliest writers on fossil plants, under the comprehensive designation “Schuppenpflanze.”

Fig. 157. Lepidodendron Veltheimianum. Ulodendron condition. (From a photograph by Dr Kidston of a specimen from the Calciferous Sandstone series, Midlothian; ⅖ nat. size.) [Kidston (02) Pl. LVII.

There has been no lack of ingenuity on the part of authors 130in offering suggestions as to the meaning of these large cup-like depressions, and there is still difference of opinion as to their significance. Lindley and Hutton[291] described them as the scars of branches or masses of inflorescence. Sir Joseph Hooker[292] speaks of a specimen of Ulodendron, shown to him by Mr Dawes, on which a large organ, supposed to be a cone, was inserted in one of the depressions, but he was unable to arrive at any conclusion as to the real nature of the fossil. While most authors have seen in the scars pressure-areas formed by the pressure of sessile cones against the surface of a growing branch, others, as for example Geinitz[293], have described the depressions as branch-scars. Carruthers[294] regarded the scars as those of adventitious roots and Williamson referred to them as the scars of reproductive shoots. The depressions vary considerably in size. The Belgian example shown in fig. 211 possesses scars 9 cm. in diameter. A specimen of Bothrodendron in the Manchester Museum from the Lancashire Coal-Measures, to which Williamson[295] has referred, bears two rows of scars 11–12 cm. in diameter on a stem 112 cm. in girth and 233 cm. long. The scars occur in two alternate series, on opposite faces of the axis, the distance between the successive scars in the same row being 29 cm. The surface-features of this large stem are not preserved.

Before considering the nature and origin of the scars it is important to remember the considerable size to which they may attain; other points of importance are the occurrence, either in the centre of each depression or in an excentric position, of an umbilicus or slightly projecting boss, in the centre of which is a pit formed by the decay of an outgoing vascular strand. The sloping sides of the scars sometimes bear elevations resembling leaf-cushions like those on the rest of the stem surface. In the specimen shown in fig. 157 the lower margin of each cup shows indistinctly the outlines of what appear to be leaf-cushions, while the rest of the sloping face is characterised by radial ridges, which may be due to bracts or leaves.

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It is obvious that in these cups we have the scars of some lateral organ, but the evidence afforded by specimens of which the depressions contain the remains of such organs is by no means conclusive. A Ulodendron has been figured by D’Arcy Thompson[296], in which the lower part of a lateral organ is attached by a narrow base to one of the scars, but the preservation is not sufficiently good to enable us to decide whether the organ is a cone or a vegetative shoot. Kidston[297] has described other examples showing portions of organs in connexion with the scars, but an examination of the specimens in his collection failed to convince me that his interpretation of them as strobili is correct.

The phenomenon known as cladoptosis, as shown on a stem of the Conifer Agathis[298] and certain Dicotyledonous trees such as Castilloa, suggests a possible explanation of the Ulodendron scars. This comparison was made by Shattock[299] in 1888, but he did not accept the resemblance as a real one. An objection may be urged to the cladoptosis hypothesis that in Ulodendron the branch, whether vegetative or reproductive, was not attached to the whole of the depressed area. On the other hand, a lateral branch originally attached by a narrow base may have continued to increase in diameter until its base became slightly sunk in the bark of the stem, thus producing a cup-like depression which, on the fall of the branch, would retain traces of the original surface-features of the stem.

Mr Watson[300] of Manchester recently published a paper on Ulodendron scars, in which he adduces fresh and, as it seems to me, satisfactory arguments in favour of the branch-scar hypothesis. Fig. 158, from one of Mr Watson’s blocks, illustrates the nature of his evidence. He points out that in the obverse half of a large specimen of Bothrodendron in the Manchester Museum, the umbilicus consists of a cylindrical hole, 18 mm. deep and 8 mm. in diameter, surrounded by a projecting ring of mineral material which doubtless represents some portion of the original plant: on the reverse half of the 132specimen the continuation of the ring is seen as a prominent cone fitting into the cup-like depression in the obverse half: the conical cast shows that numerous small vascular strands were given off from this ring of tissue, and these strands have the133 same arrangement and size as the dots which are found on typical Ulodendron scars. He interprets the ring surrounding the umbilicus as the remains of the primary wood and the small strands as leaf-traces supplying the branch.

Fig. 158. Diagrammatic section through the base of a branch to illustrate the Branch theory of the Ulodendroid scar. (After Watson.)

In the diagrammatic section shown in fig. 158 the outer cortex of the main stem is represented by oc 1; this consists of secondary tissue. The corresponding tissue in the branch is seen at oc 2. The stele of the stem is shown at Tr. St. and that of the branch at Br. St.; lt, lt, mark the position of the leaf-traces. If we assume the branch to be detached along the line LS, the depression would show numerous spirally arranged dots representing the points of exit of leaf-traces and the vascular axis would be exposed in the umbilicus. This explanation appears to me to be in harmony with the surface-features of Ulodendron scars on both Bothrodendron and Lepidodendron stems. The occasional occurrence of leaf-cushions on a portion of a Ulodendron scar is a difficulty on the cladoptosis hypothesis. Assuming that true leaf-cushions occur, their presence may, as Watson suggests, be due to the folding back of a piece of the outer cortex of the branch which has been “crushed down on to the area of the scar[301].”

Since this account was written a note has been published by M. Renier[302] in which he describes a specimen of Bothrodendron from Liège, one face of which shows a projecting Ulodendroid scar with an excentric umbilicus. On the other face is a dichotomously branched shoot with surface-features corresponding to those on the scar; the evidence that the scar represents the base of the branch is described as indisputable.

Stur[303] held the view that the depressions on Ulodendron stems represent the places of attachment of special shoots comparable with the bulbils of Lycopodium Selago, or, it may be added, with the short branches occasionally produced on Cycas stems. If the depressions were formed by the pressure of the bases of cones, it is clear that the size of the cavity must be an index of the diameter of the cone. The larger Ulodendron scars exceed in diameter the base of any known lepidodendroid strobilus. Another obvious difficulty, which has not been overlooked134 by Kidston who holds that the scars were produced by sessile cones, is that in Lepidodendron Veltheimianum strobili were borne at the tips of slender branches; the same difficulty is presented by Bothrodendron (Fig. 213). It is unlikely that two types of strobili were produced on the same plant, particularly as the cone of L. Veltheimianum was heterosporous.

The cones of certain species of Pinus remain attached to the tree for many years and their bases become embedded in the stem; this is particularly well shown in the drawing of a cone of Pinus clausa (fig. 159), for which I am indebted to Mr Sudworth, Dendrologist in the United States Forest Service. Mr Sudworth has drawn my attention to P. attenuata and P. muricata in illustration of the same phenomenon[304]. The example shown in fig. 159 cannot, however, be matched by any known specimen of Ulodendron; in the case of the depressions on the stem of a Pine the cone-base fits the circular scar, but in the fossil stems it is practically certain that this was not the case.

Fig. 159. Pinus clausa. ½ nat. size.

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There can be little doubt that certain Palaeozoic Lycopods shed their branches by a method similar to that employed by the Kauri Pine of New Zealand and by some species of Dicotyledons. The evidence adduced in the case of Bothrodendron punctatum is a strong argument in favour of extending the same explanation to other Ulodendron shoots.

Fig. 160.

1. Lepidophloios scoticus Kidst. From a specimen from the Calciferous Sandstone, Midlothian, in Dr Kidston’s Collection; rather less than ⅓ nat. size.
2. L. scoticus cone. From a specimen from the Calciferous Sandstone of Midlothian in Dr Kidston’s Collection; slightly reduced.

b. Halonia.

The branched axis with Lepidophloios leaf-cushions, represented in fig. 160, A, illustrates a special form of shoot described by Lindley and Hutton[305] under the generic name Halonia. The original specimens referred to this genus are 136decorticated axes showing remains of Lepidodendroid leaf-cushions. The spirally disposed circular scars in the specimen of Halonia (Lepidophloios scoticus[306]) shown in fig. 160 constitute the characteristic feature of the genus; they may have the form, as in fig. 160, A, of circular discs with a central umbilicus marking the position of a vascular strand, or, as in the sandstone cast of Halonia tortuosa shown in fig. 161[307], they may appear as prominent tubercles. The latter example illustrates the condition characteristic of partially decorticated stems.

Fig. 161. Halonia tortuosa L. and H. From a specimen in Dr Kidston’s Collection, from the Lower Coal-measures of Ayrshire (No. 1561); ⅔ nat. size.

In 1883 Williamson[308] described a specimen, now in the Leeds Museum, which convinced him that Halonia is merely a special form of Lepidodendron concerned with the production of fertile shoots or strobili. Feistmantel[309] also recognised that Halonia 137regularis is identical in the form of the cushions with the type known as Lepidophloios laricinus. It is worthy of note that under the name Halonia, Feistmantel[310] figured a piece of decorticated axis characterised by two rows instead of the usual spiral series of large cup-shaped scars. Recent researches have, however, tended to break down the distinction between Ulodendron and Halonia founded respectively on the biseriate and spiral arrangement of the scars or tubercles.

The interpretation of Halonial branches as cone-bearing members of Lepidodendroid plants has passed into a generally accepted statement of fact, but, so far as I know, only one specimen has been figured in which strobili are seen attached to an Halonia axis. This specimen, described by Grand’Eury[311] from the coal-field of Gard, is hardly sufficiently well-preserved to constitute a demonstration of the correctness of the generally received view, which, as is not unusual, has been repeated by one writer after another without due regard being paid to the nature of the evidence on which the statement is based. It may, indeed, be correct to describe Halonial branches as cone-bearing, but there are certain considerations which make one pause before unhesitatingly accepting this explanation. The vascular strand which passes from the central cylinder of the shoot to the tubercle or scar is composed of a solid rod of xylem distinguished from the main stele by the absence of a pith. In such petrified peduncles as have been discovered the stele is of the medullated type. The common occurrence of strobili terminating slender branches of lepidodendroid plants, though not a fatal objection to their attachment to Halonial shoots, shows that in many cases the cones were borne at the tip of leafy shoots. It may be that some of the Halonial scars are in origin like those of the Ulodendron axes of Bothrodendron and mark the position of deciduous vegetative branches.

The first account of the anatomy of Halonia we owe to Dawes[312]; this was followed by a fuller description by Binney[313]. The history of our knowledge of this type of branch has been given by Carruthers[314], who expressed the opinion that Halonia 138is merely a fertile condition of Lepidophloios and possibly of other lepidodendroid plants. He was also inclined to regard the Halonial tubercles as younger stages of the larger scars characteristic of the genus Ulodendron. Williamson’s contributions to our knowledge of Halonia are of primary importance; he supplied further proof of the Lepidodendroid nature of these branches and advanced our knowledge of their anatomy. In an early paper[315] he expressed the view that the differences on which Halonia and Ulodendron are separated are such as result from a difference in age and are not of generic importance. In the last memoir, of which he was sole author, published by the Royal Society[316], Williamson brought forward further evidence in support of this well-founded opinion.

That the fossils known as Halonia are branches of a lepidodendroid plant is at least certain, and it is probable that the lateral branches which they bore were fertile, though satisfactory proof of this is lacking. We know also that Halonia branches are characterised by the Lepidophloios form of leaf-cushion; there is, however, no sufficient reason to assume that such branches were never attached to stems with the cushions of the Lepidodendron form. The further question, namely whether Williamson was correct in his contention as to the absence of any essential distinction between Ulodendron and Halonia, does not admit of an unchallenged answer. In 1903 Weiss[317] described the anatomy of a specimen of a biseriate Halonia branch of Lepidophloios. The form of the leaf-cushions is unfortunately not very well preserved, but Weiss figures other specimens with two rows of tubercles on which the leaf-cushions are sufficiently distinct to justify a comparison with those of Lepidophloios. He believes with Williamson that it is the presence of tubercles in place of scars which distinguishes Halonia from Ulodendron, and that the arrangement of the tubercles or scars is a matter of little importance. He expresses the opinion justified by the evidence available that the absence or presence of tubercles is merely due to accidents of preservation or, one may add, to difference in age. Kidston[318] dissents from Weiss’s description of his specimen as a biseriate 139Halonia; he regards it as a Ulodendron branch of Sigillaria discophora (König). Until specimens with more clearly preserved external features are forthcoming it is impossible to settle the point in dispute, but on the facts before us there would seem to be a prima facie case in favour of Weiss’s contention.

The designation Halonia may be retained as a descriptive term for Lepidodendroid shoots characterised by spirally disposed scars or tubercles and bearing leaf-cushions of the Lepidophloios type. In the case of specimens showing prominent tubercles, the superficial tissues are usually absent and, as in the fossil represented in fig. 161, the name Halonia does not necessarily imply the presence of leaf-cushions of a particular type.

vii. Anatomical characters of Vegetative Lepidodendron shoots (Lepidodendron and Lepidophloios).

The type already described under the name Lepidodendron vasculare differs from those dealt with in the following pages chiefly in the anatomy of the stele. The simplest and probably most primitive type of Lepidodendron stem is that in which the xylem forms a solid rod; the type of stele most frequently represented is that of L. Harcourtii, L. fuliginosum, and other species in which the diameter of the stele is greater and a cylinder of primary xylem encloses a comparatively large parenchymatous pith.

1. Lepidodendron esnostense, Renault[319].

This species was founded by Renault on petrified specimens from the Culm beds of Esnost in France. The surface of a young twig bears prominent leaf-cushions of elongated rhomboidal form similar to those of Lepidodendron obovatum (fig. 173) and other species. In older branches the primary cortex is replaced by a considerable thickness of radially disposed secondary cortical tissue which, as shown in tangential section, consists of a reticulum of elongated pointed elements 140with comparatively thick walls enclosing meshes filled with large-celled parenchyma. It is worthy of note that if such a branch were exposed to decay, the earlier destruction of the more delicate tissue in the meshes of the secondary cortex would produce a series of oval depressions, corresponding to the parenchymatous areas, separated by a projecting reticulum of the more resistant elements: a cast of this partially decayed surface would be indistinguishable from that of some types of Sigillaria or of a Lyginodendron. The inner regions of the cortex of the type-specimens have not been preserved. The xylem, which is the only part of the stele represented, has the form of a protostele or solid cylinder of scalariform tracheids with peripheral groups of narrower protoxylem elements which mark the points of exit of the leaf-traces: in a branch 1–2 cm. wide the xylem column has a diameter of 3 mm. The small leaves (fig. 143, B, C), similar to those of a Sigillaria, are sub-rhomboidal in section near the base and approximately circular near the apex[320]. The mesophyll consists of palisade cells having the appearance of typical chlorophyll-tissue. The heterosporous strobili attributed to this species bore microsporangia on the upper and megasporangia on the lower sporophylls; the megaspores, of which a considerable number occur in each megasporangium, are identical in size with those of another Culm form, Lepidodendron rhodumnense. Some of these have retained traces of prothallus tissue, and in one spore Renault figures what he regards as an archegonium: the drawing is by no means convincing.

2. Lepidodendron rhodumnense, Renault[321].

The species from the Culm of Combres (Loire) agrees in its solid xylem cylinder and in the differentiation of the secondary cortex, as also in the association of two kinds of spore, with Lepidodendron esnostense. A comparison of the leaves of the two types reveals certain differences which may be of specific rank, but, apart from minor differences, these Culm species may be classed under one anatomical type.

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3. Lepidodendron saalfeldense, Solms-Laubach[322].

This Devonian species was founded on a specimen 3 × 2·5 cm. broad at the base, which shows the stumps of four branches recalling the dichotomously branched arms of Stigmaria and Pleuromeia. If these are in reality the remains of Stigmaria-like horizontal branches the species affords an interesting example of a Lepidodendron axis with a subterranean rhizome of the type which has been found in several Sigillarian stems. In the upper end of the axis the stele consists of a solid strand of xylem which is not sufficiently well preserved to show the position of the protoxylem groups. A transverse section taken near the base reveals a type of stele differing from that at the upper end in being composed of radially disposed tracheids and in its resemblance to the stele of Stigmaria.

4. Lepidodendron fuliginosum, Williamson. Figs. 162–172, 179, E.

The name Lepidodendron fuliginosum was proposed by Williamson in 1887 for petrified stems previously included by him in Witham’s species L. Harcourtii, but subsequently recognised as a distinct type characterised by “the greater uniformity in the composition of the entire cortex” and by other features some of which do not constitute distinctive characters. The species agrees with L. Harcourtii and with L. Veltheimianum in having a medullated stele; it is distinguished not 142only by the more frequent preservation of the middle cortex, a fact due to a difference in minute structure, but chiefly by the peculiar structure of the secondary tissue added to the stele; this is in part composed of radial series of parenchymatous cells and of a varying amount of tracheal tissue the elements of which are narrower than in other species and are characterised also by their sinuous vertical course. As is pointed out in the sequel, the anatomical features of L. fuliginosum, as at present understood, are not confined to one type of Lepidodendron stem. Specimens have been described with leaf-cushions of the form characteristic of L. aculeatum, L. obovatum and Lepidophloios combined with the anatomical features of Williamson’s species: it is possible that the two species L. obovatum and L. aculeatum are not really distinct[323], but it is certain that shoots with both the Lepidodendron and Lepidophloios cushions may have the same type of anatomical structure.

A more detailed knowledge of the structural features of Lepidodendron shoots may enable us to define anatomical species with more exactness than is possible at present. There can, however, be little doubt that well-marked anatomical features may be associated with more than one specific form of shoot as defined by the form of the leaf-cushions.

Solms-Laubach proposed the name Lepidodendron Williamsoni for the anatomical type L. fuliginosum of Williamson, but the latter name has been generally adopted.

In the following account special attention is directed to the nature and origin of the secondary stelar tissue and to the secretory zone, as difference of opinion exists as to the interpretation of these features. Among the best examples of shoots of Lepidodendron fuliginosum without secondary tissue or in which it is feebly developed are those originally described by Binney. The stele includes a large parenchymatous pith, the cells of which frequently show signs of recent division, a feature observed also in the pith of the large stem of L. Wünschianum, represented in figs. 181, 182. The primary xylem cylinder has an irregularly crenulate outer edge like that of L. Wünschianum 143and L. Harcourtii and the protoxylem elements occupy an exarch position. Isodiametric reticulately-pitted elements are met with both on the inner and outer edge of the xylem.

Fig. 162. Lepidodendron fuliginosum. Part of the stele in transverse section. (Binney Collection, Sedgwick Museum, Cambridge.)

Fig. 163. Lepidodendron fuliginosum. Longitudinal section. (Binney Collection, Cambridge.)

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Figs. 162 and 163 illustrate the structure of the outer portion of the xylem and adjacent tissues in a section of a shoot 3·8 cm. × 2·5 cm. in diameter, which is in the act of branching, as shown by the occurrence of two steles of equal size. A figure of the complete section will be found in Binney’s memoir[324], and additional illustrations were published in 1899[325].

Fig. 164. Lepidodendron fuliginosum. Leaf-trace. (Binney Collection, Cambridge.)

The primary xylem (figs. 162, 163, x) is succeeded by 2–3 rows of polygonal cells with dark contents and associated with isodiametric tracheae: these pass into clearer parenchymatous tissue, a, characterised by the arrangement of the cells in vertical series, to which the term meristematic zone has been applied. The secretory zone, s, abutting on the meristematic zone, consists of more or less disorganised parenchymatous cells 145and broader and more elongated spaces; it is interrupted here and there by an outgoing leaf-trace, as at lt 1 and lt 2 in fig. 162. The secretory zone is succeeded by a homogeneous inner cortex like that described in L. vasculare; part of this region is seen at the upper edge of fig. 162. The broad middle cortex, which is separated from the inner cortex by a sharply defined boundary, is composed of rather small lacunar parenchymatous tissue consisting of sinuous tubular elements interspersed among isodiametric cells of various sizes (fig. 166, p). In the middle cortical region the leaf-traces pursue an almost horizontal course; one is shown in fig. 164, in oblique longitudinal section, in a reversed position; the xylem, x, should be on the inner side of the secretory tissue, s. The clear space between the two parts of the vascular bundle was originally occupied by a few layers of parenchymatous cells, as seen in the transverse sections, figs. 165 and 166. In some specimens the leaf-traces pass through the middle cortex in a much more vertical course, as shown by the section represented in fig. 165. This section illustrates the structure of a typical leaf-trace with unusual clearness; it shows the tangentially elongated group of xylem, the strand of tissue which occupies the position of phloem,146 s (to which the term secretory zone is applied), the compact parenchyma between the two parts of the bundle, and surrounding the whole a narrow sheath sharply contrasted by the smaller and more uniform size of the cells from the middle cortex, a few cells of which are seen in the photograph. The middle cortex shows a well-defined junction with the more compact outer cortical region, which consists of primary parenchyma passing externally into a zone of phelloderm composed of thick-walled and more elongated cells. A noticeable feature in many Lepidodendron shoots is the occurrence of a circle of strands of secretory cells often surrounding fairly large ducts just internal to the edge of phelloderm: similar strands form irregularly concentric circles, as was pointed out in the case of L. vasculare, in the phelloderm itself.

Fig. 165. Lepidodendron fuliginosum. Leaf-trace: x, xylem; s, secretory zone. (Binney Collection, Cambridge.)

Fig. 166. Lepidodendron fuliginosum. Leaf-trace: p, parichnos. (Binney Collection, Cambridge.)

Fig. 166 shows a leaf-trace in the outer cortex accompanied by its crescent-shaped parichnos, p, derived from the middle cortex and by means of which the outer cortex and the lamina of the leaves are connected with the inner region of the shoot. This lacunar middle cortex and parichnos doubtless constitute an aerating tissue-system which after leaf-fall is exposed directly to the air at the ends of the parichnos arms on the leaf-scars.

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Some of the sections in the Binney Collection (Sedgwick Museum, Cambridge) show early stages in the production of secondary xylem: in the section represented in fig. 167 the secretory zone is succeeded on its inner face by a zone of radially elongated cells, m, which are clearly in a meristematic condition. The same section shows also the more radially extended form of the xylem of a leaf-trace with its internal protoxylem, px, in contrast to the tangentially elongated form which is assumed during its passage through the cortex (cf. figs. 165, 166).

Fig. 167. Lepidodendron fuliginosum. (Binney Collection, Cambridge.)

Some sections of Lepidodendron fuliginosum in the Manchester University Collection are of special interest from the point of view of the method of secondary thickening. In the section reproduced in fig. 168, B, the meristematic zone is seen to consist in part of radially elongated elements, m, with parallel cross-walls evidently of recent origin. The same tissue is shown also in fig. 168, C, a, D, a, and in fig. 169, A, a148 This band of meristem, which we may speak of as the cambium, occurs in the outer region of the meristematic zone immediately internal to the secretory zone, sc.

Fig. 168.

1. Lepidodendron vasculare. (Botany School, Cambridge.)
2. Lepidodendron fuliginosum. (From a specimen from Shore, Lancashire, in the Cambridge Botany School Collection).
3. L. fuliginosum. (“Biseriate Halonia” of Weiss No. 257, Manchester University Museum.)
4. L. fuliginosum. (Manchester Univ. Museum.)

The result of the activity of this cambium band is the production of secondary parenchyma and tracheal tissue. In fig. 179, E, drawn from a portion of the section represented in fig. 168, B, a projecting arm of primary xylem is seen at x; this is followed by 2–3 layers of parenchymatous cells, some of which have dark contents, and beyond this is seen a group of secondary elements, tr, cut across somewhat obliquely, which are evidently products of the cambial cells on the inner margin149 of the secretory zone, sc. The longitudinal section (fig. 169, D) shows the cambial cells, a, next the secretory zone, sc, passing internally into crushed and imperfectly preserved elongated elements which are presumably miniature tracheae, and these are succeeded by older and more completely lignified xylem elements, x. In larger shoots the amount of secondary tissue is considerably greater; it may consist almost entirely of short-celled parenchyma (fig. 168, C, from x to sc), or it may include a large proportion of radially disposed and vertically elongated 150tracheae (fig. 168, D, x², and fig. 170, A, x²), or it may consist of parenchyma containing scattered groups of tracheae (fig. 169, A, x²)[326].

Fig. 169. Lepidodendron fuliginosum.

• A, B. (Manchester University Collection. No. Q. 645 A.)
• B, C. (Manchester. No. 257.)
• D. (Manchester. No. 6.)

Fig. 169, A, is a diagrammatic sketch of the tissues—1 mm. wide—between the primary xylem, x, and the inner cortex. The primary xylem is succeeded by short parenchymatous cells followed by a zone of radially elongated elements passing occasionally into rows of narrow scalariform tracheae, some of which, owing to their sinuous longitudinal course (fig. 171, C), are seen in oblique section, as at C, fig. 169, A. At its outer edge this secondary tissue, x², consisting of parenchyma and tracheae, passes into the cambial band (fig. 169, B, a).

Fig. 170. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)

The radial longitudinal section represented in fig. 168, C, is taken from the fossil described by Weiss as a biseriate Halonia; it agrees sufficiently closely in structure with others referred to Lepidodendron fuliginosum to be classed as an example of this anatomical type. A complete transverse section of the stem measures 9 × 6·3 cm.; the breadth of the tissues between the edge of the primary xylem and the outer edge of the secretory zone is 2·5 mm. The middle cortical region, characterised by the sooty appearance, which led Williamson to choose the specific name fuliginosum, is traversed by the leaf-traces and is sharply differentiated from both the inner and outer cortex. 151The longitudinal section (fig. 168, C) shows the outer edge of the primary xylem, x, abutting on a band of dark and small-celled parenchyma which passes into the broad zone of secondary tissue, m, the inner region of which consists of fairly thick-walled elements in radial series passing externally into the thin-walled cells of the cambial region, a, on the inner edge of the secretory zone, sc. This section shows also the interruption of the secretory zone by an outgoing leaf-trace, lt, the lower part of which, sc, is continued downwards into the secretory zone. The exit of a leaf-trace produces a gap in the secretory zone of the stem, but not in the xylem. If we applied the term phloem to the secretory zone—a course adopted by Prof. F. E. Weiss and some other authors, but which I do not propose to follow—we should speak of a phloem foliar-gap as a characteristic feature of a152 Lepidodendron shoot. This applies to other species of the genus as well as to L. fuliginosum.

Fig. 171. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)

Fig. 171, A, shows more clearly the broad zone of secondary parenchyma with the thinner-walled cambial region, a; the latter is represented on a larger scale in fig. 171, B. The section shown in fig. 168, D, and in fig. 170, A, affords an example of a stem in which the secondary tissue consists largely of narrow scalariform tracheae, x²; the primary stele has a diameter of 1 cm.; the secondary xylem, x², forms a fairly broad zone of parenchyma and tracheal elements through which leaf-traces pass vertically, a fact of some interest in comparison with the horizontal course which they pursue through the medullary rays in the normal secondary wood of L. vasculare and L. Wünschianum. The secondary tracheae pass gradually into thin-walled cambial cells (a, fig. 168, D; 170, A) with parallel tangential walls. Fig. 171, C, shows the sinuous course of the secondary tracheae as seen in longitudinal section, and a few small groups of parenchymatous cells, mr, which may be of the nature of medullary rays, enclosed between the winding scalariform tracheae.

Fig. 172. Lepidodendron fuliginosum. From a section (4 × 3·4 cm.) in the Williamson Collection, British Museum (No. 379), figured by Williamson, Phil. Trans. R. Soc. 1881, Pl. 52.

The secretory zone of Lepidodendron fuliginosum agrees essentially with that of other species; it usually presents the153 appearance shown in fig. 168, B, sc; fig. 169, B and C; fig. 170, B (longitudinal section); fig. 171, D, sc. The comparatively large clear spaces which characterise this tissue, as seen in fig. 168, B, appear to owe their origin to groups of small cells which gradually break down and give rise to spaces containing remnants of the disorganised elements, as in fig. 171, D, and fig. 169, B, b. The secretory tissue seen in fig. 170, B, consists of large and small parenchymatous cells without any of the broad sacs or spaces such as are shown in fig. 169, C.

Fig. 172 represents a diagrammatic sketch of a transverse section (4 × 3·4 cm. in diameter) of a young shoot from the Lower Coal-Measures of Lancashire figured by Williamson[327] in 1881 as Lepidodendron Harcourtii. It shows the features characteristic of L. fuliginosum and is of importance as affording an example of a shoot giving off a branch from the stele to supply a lateral axis of the type characteristic of Halonia. The exit of the branch-stele forms a gap in the main stele; a ramular gap as distinguished from a foliar gap. The outgoing vascular strand is at first crescentic, but becomes gradually converted into a solid stele. The primary xylem of the main stele (black in the figure) consists of a ring six tracheae in breadth; this is succeeded by a few layers of dark parenchymatous cells and a band of radially elongated elements, a, which abuts on the secretory zone. The middle lacunar cortex, c², with Stigmaria rootlets, s, is fairly well preserved. In the outer cortex occur several leaf-traces, lt, accompanied by spaces originally occupied by the parichnos strand, p. A band of secondary cortex, consisting chiefly of phelloderm, is seen at pd. The prominent leaf-cushions, some of which show the parichnos, p, appear to be of the Lepidophloios type.

It remains to consider the external characters of Lepidodendroid shoots possessing the anatomical features represented by the comprehensive species Lepidodendron fuliginosum.

Certain sections exhibiting this type of structure were described by Binney in 1872 as Halonia regularis[328] on evidence supplied by Mr Dawes, who stated that they were cut from a 154specimen bearing Halonia tubercles. The section represented in fig. 172 is no doubt from an Halonia axis. In 1890 Cash and Lomax[329] stated that they had in their possession a stem of the L. fuliginosum type with the external features of Lepidophloios; this identification has been confirmed by Kidston[330] and Weiss[331]. It is, however, equally clear that certain species with the elongated leaf-cushions of Lepidodendron must be included among examples of shoots with the anatomical characters of L. fuliginosum.

Fig. 173. Lepidodendron obovatum. (From a specimen lent by Dr D. H. Scott.)

Dr Scott[332] published in 1906 a short account of the structure of a specimen from the Lower Coal-Measures of Lancashire, the external features of which were identified by Kidston with those of Lepidodendron obovatum Sternb. Dr Scott generously allowed me to have drawings made from his specimen; these are reproduced in fig. 173. The form of the leaf-cushion is by no means perfect; there is a well-marked median ridge, and the small circular scar near the upper end of some of the cushions may represent the ligular cavity. At the base of the leaf-cushions a cortical meristem has produced a zone of 155secondary cortex; at c a second meristem is seen in the outer cortex: the dark dots in the cortex mark the positions of leaf-trace bundles. The inner cortex, d, is a more compact tissue156 surrounding the imperfectly preserved secretory zone. From the medullated stele a lateral branch, b, is being given off; its crescentic form becoming changed to circular as it passes nearer to the surface.

Fig. 174. Lepidodendron aculeatum. (Cambridge Botany School.)

Fig. 175. Lepidodendron aculeatum. (Cambridge Botany School.)

A type of Lepidodendron, L. Hickii, founded on anatomical characters by Mr Watson[333], is believed by him to possess leaf-cushions like those of L. obovatum; if this is so, it is interesting, as he points out, to find two distinct anatomical types associated with one species. Watson thinks it probable that the “species” L. obovatum includes at least two widely different species. This merely emphasizes the importance of correlating structure and external characters as far as available data permit.

Fig. 176. Lepidodendron aculeatum. (Cambridge Botany School.)

The specimen, of which part of the surface is shown in fig. 174, is in all probability L. aculeatum Sternb. This was described by me in detail in The Annals of Botany (1906) as another example of the co-existence of the Lepidodendron fuliginosum type of anatomy with a true Lepidodendron. The locality of the specimen is not known. The leaf-cushions are 1·5 cm. long with tapered upper and lower ends; a ligular cavity may be recognised on some parts of the fossil, also faint indications of leaf-trace scars. The tubercles (fig. 174, A–C, t) probably represent leaf-traces which the shrinkage of the superficial tissues has rendered visible in the lower part of their course. The circular scar, s (fig. B), on the partially decorticated surface is apparently a wound. The stele is sufficiently 157well preserved to justify its reference to L. fuliginosum. The irregularly crenulated edge of the primary xylem, x (fig. 175), is succeeded by a broad band of parenchyma (the meristematic zone), m, and beyond this are remnants of the secretory zone, s. The structure of the leaf-traces corresponds with that of other specimens of the type, but the much steeper course of these vascular strands, lt, lt′ (fig. 176), is a feature in which this example differs from most of those referred to L. fuliginosum. Such evidence as is available would seem to point to the absence of trustworthy criteria enabling us to separate, on anatomical grounds, Lepidophloios and Lepidodendron[334].

Fig. 177. Stigmaria radiculosa (Hick). (From sections in the Manchester University Collection.)

Stigmaria radiculosa (Hick).

We have no proof of the nature of the subterranean organs of Lepidodendron fuliginosum, though it is not improbable that the specimens described below may be correctly assigned by Weiss to that species. Prof. Weiss[335] has made an interesting contribution to our knowledge of a type first described by Hick[336] under the name Tylophora radiculosa, a designation which he afterwards158 altered to Xenophyton radiculosum[337] and for which we may now substitute Stigmaria radiculosa (Hick). Prof. Williamson expressed the opinion that Xenophyton exhibited considerable affinity with Stigmaria ficoides and Weiss’s further study of the species leads him to regard Hick’s plant as probably the Stigmarian organ of Lepidodendron fuliginosum. The diagrammatic transverse section represented in fig. 177, A (4·5 cm. in diameter), shows an outer cortex of parenchyma, c³, consisting in part of radial rows of secondary tissue and of a band of compact parenchyma bounded by the wavy line a; at sc is a series of secretory strands exactly like those in a corresponding position in Lepidodendron fuliginosum and other species of the genus. The greater part of the organ is occupied by a lacunar and hyphal middle cortex identical in structure with that shown in fig. 178, B, drawn from a rootlet. At d, fig. 177, A, the middle cortex has been 159invaded by a narrow tongue of outer cortical tissue. The stele is characterised by a large pith filled with parenchyma; in Stigmaria ficoides[338] the general absence of pith-tissue has led to the inference that the stele was hollow. The xylem is represented by a ring of bundles separated by broad medullary rays; each bundle contains a few small, apparently primary, elements on its inner edge but is mainly composed of radial rows of secondary tracheae x², fig. 177, B. On the outer face of the secondary xylem occur a few smaller and thinner walled cells, c, having the appearance of meristematic tissue; from these additional tracheae were added to the xylem. This meristematic zone occurs, as in the stems of Lepidodendron, immediately internal to the secretory tissue, sc; at c¹, fig. 177, B, is seen the inner cortical tissue.

Fig. 178. Rootlet of Stigmaria. (From a section in the Manchester Collection.)

In surface-view a specimen figured by Hick[339] shows a number of circular scars agreeing in shape and arrangement with the rootlet scars of Stigmaria ficoides. At b in fig. 177, A, the basal portion of a rootlet is shown in organic connexion with the outer cortex. The rootlet-bundles are given off from the stele as in other examples of Stigmaria; each bundle consists of a triangular strand of xylem with an endarch protoxylem at the narrow end accompanied by a portion of the secretory tissue as in the leaf-traces. As in Stigmaria ficoides the rootlets are attached to the outer cortex above a cushion of small cells. It is interesting to find that rootlet-bundles, as seen in tangential section of the main axis, are associated with a parichnos strand, but this is on the xylem side of the vascular strand, whereas in the case of leaf-traces the parichnos is on the other side of the bundle.

Fig. 178, A, represents a transverse section of a rootlet (6 mm. in diameter) associated with Stigmaria radiculosa and probably belonging to this species. The xylem strand x is composed of a group of tracheae with a single protoxylem strand, px, at the pointed end and with small metaxylem elements at the broad end next the space originally occupied by the so-called phloem. A parenchymatous sheath, c′, surrounds the bundle, and beyond this is the broad middle cortex, a small portion of which is shown on a larger scale in fig. 178, B; as Weiss 160points out, some of the outermost cells of the lacunar cortex (m) are clearly in a state of meristematic activity.

The preservation of the middle cortex and the small quantity of secondary xylem are characters which this Stigmaria shares with Lepidodendron fuliginosum, and although decisive evidence is still to seek, we may express the opinion that Weiss’s surmise of a connexion between Stigmaria radiculosa and Lepidodendron fuliginosum is probably correct.

5. Lepidodendron Harcourtii. Fig. 179, A–D.

In 1831 Mr Witham[340] published an anatomical description of a fragment of a Lepidodendron which he named Lepidodendron Harcourtii after Mr C. G. V. Vernon Harcourt from whom the specimen was originally obtained. The fossil was found in rocks belonging to the Calciferous series in Northumberland. Witham reproduced the account of this species in his classic work on Fossil Vegetables[341], and Lindley and Hutton[342], who examined Mr Harcourt’s material, published a description of it in their Fossil Flora in which they expressed the view that Lepidodendron is intermediate between Conifers and Lycopods. Adolphe Brongniart[343] included in his memoir on Sigillaria elegans an account of Witham’s species based on material presented to the Paris Museum by Mr Hutton and Robert Brown. Dr Kidston[344] has shown that the actual transverse section figured by Witham is now in the York Museum; a piece of stem in the same Museum, which is not the specimen from which Witham’s section was cut, supplied the transverse section figured by Brongniart. The figures given by Lindley and Hutton do not appear to have been made from the York specimens. In 1887 Williamson[345] published a note in which he pointed out that some of the specimens described by him as L. Harcourtii should be transferred to a distinct species, which he named L. fuliginosum. Subsequently in 1893 he gave a fuller account of Witham’s species; it has, however, been shown by Dr Kidston and by 161Mr Watson[346] that certain specimens identified by Williamson as L. Harcourtii differ sufficiently from that type to be placed in another species, for which Watson proposes the name L. Hickii.

A paper on L. Harcourtii published by Bertrand[347] in 1891 extends our knowledge of this type in regard to several anatomical details. It was recognised by Williamson that the absence of secondary wood in shoots possessing the anatomical characters of L. Harcourtii is a feature to which no great importance should be attached. It is possible that the large stems from the Isle of Arran described by Williamson[348] as Lepidodendron Wünschianum, in which the secondary wood is well developed, may be specifically identical with the smaller specimens from Northumberland and elsewhere which are recognised as examples of Witham’s type.

The diagrammatic sketch shown in fig. 179, A, was made from a section figured by Williamson in 1893[349]; it has a diameter of 9 × 8·5 cm. The stele is of the medullated type like that of L. Wünschianum, and the outer edge of the primary xylem is characterised by sharp and prominent projecting ridges similar to those of L. fuliginosum but rather more prominent. Parenchymatous cells succeed the xylem, as in other species, but in this case there is no indication of meristematic activity; beyond this region occur occasional patches of a partially destroyed secretory zone. Remains of a lacunar tissue are seen in the middle cortical region; also numerous leaf-traces, lt, consisting of a tangentially elongated xylem strand accompanied by a strand of secretory zone tissue enclosed in a sheath of delicate parenchyma. In the inner part of the outer cortex, c³, the leaf-traces lie in a space originally occupied by the parichnos; in the outer portion of the same region a band of secondary cortex, pd, has been formed; immediately internal to this occur numerous patches of secretory tissue, represented by small dots in the drawing close to pd; one is shown on a larger scale in fig. B.

The position of the phellogen is seen at a; external to this are radial rows of rather large cells with dark contents.

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Fig. 179.

• A–D. Lepidodendron Harcourtii, Witham.
• E. Lepidodendron fuliginosum, Shore, Lancashire.
• A, B. From a specimen in the Williamson Collection, British Museum (No. 380), from Airdrie, Scotland.
• C, D. From sections in the Collection of Dr Kidston, from Shore, Lancashire.

Fig. 179, C, x, shows the characteristic form of the primary xylem edge, beyond which are seen oval or circular leaf-traces with a mesarch protoxylem, lt, px. It is possible that this specimen may not be specifically identical with Witham’s species, but it represents a very similar if not identical type; it may on the other hand be referable to L. fuliginosum. The importance of the specimen, apart from its precise specific position, is that it serves to illustrate the general appearance of the xylem surface met with in both species, L. Harcourtii and L. fuliginosum. A tangential longitudinal section, taken through the line ab in fig. C, is represented in fig. 179, D. The xylem of the leaf-traces lt, consisting chiefly of scalariform tracheae, alternates with patches of crushed and delicate parenchyma which163 immediately abut on the primary xylem; at p, p, the section passes through some of the projecting arms of the xylem cylinder; at m is seen a patch of meristematic zone tissue. This section together with the similar section of Lepidodendron vasculare described on a previous page demonstrates that the projecting ridges of the primary xylem form apparently vertical bands: they are not characterised by a lattice-work arrangement as described by Bertrand and by other authors who have accepted his conclusions. If a reticulum of intersecting ridges were present on the face of the xylem cylinder its existence would be revealed by such a section as that represented in fig. 179, D.

Fig. 180. Lepidodendron Wünschianum. From Arran. (⅕ nat. size.) (Sedgwick Museum, Cambridge.)

6. Lepidodendron Wünschianum (Williamson). Figs. 180–184.

Reference was made in Volume I. to the occurrence of large stems of a Lepidodendron in volcanic beds of Calciferous sandstone age in the island of Arran[350]. These were discovered and briefly described by Mr Wünsch in 1867[351] and afterwards named by Carruthers Lomatophloyos Wünschianus[352]. Mr Carruthers 164visited the locality and published an account of the peculiar method of preservation of the plant remains[353]. It is, however, to Williamson[354] that we owe the more complete description of these Arran stems. Portions of large stems from the Arran beds are preserved in the British Museum, the Sedgwick Museum, Cambridge, and in the Manchester Museum. The section of one of these is shown in fig. 180; an outer shell of bark encloses a mass of volcanic ash in which are embedded several woody cylinders originally described as “internal piths[355],” and by Carruthers as young stems produced from spores which had germinated in the hollow trunk of a large tree. The true interpretation was supplied by Williamson who showed that a stem of the dimensions of that represented by the outer cortex, e, fig. 180, must have possessed a single stele of the size of those seen in the interior of the hollow trunk. The additional woody cylinders, or steles, were derived from other stems, and carried, probably by water, into the partially decayed trunk. In addition to large Lepidodendron stems Williamson described smaller shoots as well as an Halonial branch and made brief reference to some cones described by Binney[356] in 1871 from the same locality.

The following account of Lepidodendron Wünschianum is based on an exceptionally fine specimen discovered by Mr T. Kerr of Edinburgh in Calciferous sandstone volcanic ashes at Dalmeny in Linlithgowshire. The material from this locality described by Mr Hill and myself[357] was generously placed in my hands by Dr Kidston of Stirling. Fig. 181, A, shows a transverse section, 33 cm. in diameter, consisting of a shell of outer cortical tissue enclosing a core of light-coloured volcanic ash; on the decay of the more delicate middle cortex the cylindrical stele dropped to one side of the hollow trunk. The stele, fig. 182, has a diameter of 6·5 cm.; the centre is occupied by concentric layers of silica, s, surrounded externally by the remains of a parenchymatous pith, p, made up of isodiametric and sinuous hypha-like elements like those in the middle cortex of Lepidodendron shoots. On the inner edge of the primary 165xylem, x′, occur several isodiametric tracheae with fine scalariform and reticulate thickening bands like those in the central region166 of the stele of Lepidodendron vasculare: it is probable that these elements are vestiges of conducting tissue which in ancestral forms formed a solid and not a medullated stele.

Fig. 181. Lepidodendron Wünschianum. Calciferous Sandstone, Dalmeny. (A, Sedgwick Museum, Cambridge. B–F, Botany School, Cambridge.)

Fig. 182. Lepidodendron Wünschianum. The stele of the stem shown in fig. 181, A. (Cambridge Botany School.)

The primary xylem is limited externally by an unequally fluted surface with exarch protoxylem elements; it is, however, noteworthy that there is not always a very clearly defined difference between the small protoxylem and the large centripetally developed tracheae. Immediately beyond the primary xylem occur numerous thin-walled parenchymatous cells with spiral and reticulate pitting; beyond these is the broad zone of167 secondary xylem, x², composed of scalariform tracheae and numerous medullary rays consisting of one, two, or several rows of radially elongated elements with spiral and reticulate pitting. In tangential sections the rays are seen to vary considerably in size, some being made up of a single row of cells while others are longer and broader; through the latter leaf-traces pass horizontally. Portions of medullary rays are seen at mr in fig. 181, C and E.

The leaf-traces given off from projecting ridges on the outer edge of the primary xylem pass upwards for a short distance and then bend outwards through a broad medullary ray; on reaching the limit of the secondary xylem they again bend sharply upwards, appearing in transverse section at lt fig. 181, B. Each leaf-trace consists at first of long tracheae accompanied by numerous thin-walled spiral and reticulate parenchymatous cells derived from the tissue in contact with the outer edge of the primary wood. Fig. 181, B, shows a leaf-trace near the edge of the secondary xylem; it consists of a group of primary tracheae, with narrower protoxylem elements, px, near the outer margin, almost completely enclosed by radially disposed series of smaller and more delicate tracheae. These secondary elements of the leaf-trace are apparently added during its passage through the medullary ray, but additions are also made to this tissue by the meristematic zone, m, fig. 181, B and E. In contact with the outermost tracheae of normal size at the edge of the secondary xylem there are some smaller lignified elements, as at a, fig. 181, E, and at T, fig. 183; this juxtaposition of large and small tracheae has been referred to in the description of L. vasculare.

Prof. Williamson[358], in his account of the Arran specimens of this species, expressed the opinion that the trees probably perished “in consequence of the mephitic vapours which filled the atmosphere”; it maybe that in the striking difference in the diameter of the conducting elements on the margin of the wood we have evidence of approaching death.

Beyond the most recently formed tracheae we have a band 168of delicate parenchymatous cells (m, figs. B and E, 181; C, figs. 183, 184) forming the meristematic zone[359]. The longitudinal section represented in fig. 184 shows some recently formed narrow tracheae, T, and beyond these the meristematic zone composed of thin-walled short cells, C, arranged in horizontal rows. It is this small-celled tissue to which the name phloem has been applied by some authors[360], a term which seems to me to be misleading and inappropriate. In passing through this zone of dividing cells the leaf-traces become surrounded by an arc of meristem from which elements are added to the radially placed rows of secondary tracheae. Beyond the meristematic region portions of the 169secretory zone are preserved, consisting of large sacs or spaces and small dark cells as seen in figs. 181, B, E, sc, F; 183, 184. This tissue has the same structure as in L. vasculare and in L. fuliginosum: it is a striking fact that there are no indications of any additions to the secretory zone even in stems with such a large amount of secondary xylem as in the Dalmeny specimen (fig. 182, x²). If the secretory zone were of the nature of phloem we should expect to see signs of additions made to it in the course of growth. In this connexion it is worth mentioning that in the recent fern Botrychium (Ophioglossaceae) secondary xylem is formed in the stem, but apparently no additions are made to the phloem. The structure of the secretory zone tissue as seen in the longitudinal section fig. 184, S, is also a serious difficulty in the way of accepting the designation phloem as170 employed by Scott and Weiss. Between the secretory zone and the outer cortical region, no tissues have been preserved. The shell of bark consists chiefly of radial rows of elongated cells with rather thick walls characterised by the occurrence of small intercellular spaces and by tangentially placed bands of secretory cells and sacs (fig. 181, D, s). Immediately internal to the secondary cortex or phelloderm occur groups of secretory tissue as shown in the section of L. Harcourtii (fig. 179, B).

Fig. 183. Lepidodendron Wünschianum. From the specimen shown in fig. 181, S, secretory zone; C, meristem; T, immature tracheae.

Fig. 184. Lepidodendron Wünschianum. Longitudinal section of the specimen represented in transverse section in fig. 183.

The large tree shown in transverse section in fig. 181, A, has lost its leaf-cushions; the bark, as seen in the lower part of the photograph, presents a fissured appearance like that with which we are familiar on an old Oak or Elm stem. A radial longitudinal section through the phelloderm revealed the existence of a crushed leaf-trace passing outwards in an approximately horizontal course accompanied by a strand of parenchymatous tissue[361] having the characteristic structure of a parichnos. It is probable that the surface of this partially decorticated stem differed in appearance from that of an old Sigillaria (cf. fig. 198) in the much smaller and less conspicuous parichnos strands.

In addition to the large stems of L. Wünschianum from Arran and Dalmeny numerous examples of smaller axes from the former locality are represented in the Williamson collection (British Museum). Some of the twigs are characterised by a solid stele (protostele) giving off numerous leaf-traces accompanied by short spirally thickened tracheids like those which occur at the outer edge of the primary xylem in the larger stem: these extend into the leaf where they are arranged round the vascular bundle like the transfusion tracheids[362] in many recent conifers. The surface of these smaller shoots bears large leaf-cushions which are seen in longitudinal section to have the form characteristic of Lepidophloios. It is worthy of note that a section of a bifurcating axis of this species from the Calciferous Sandstone of Craigleith (British Museum Collection[363]), although its diameter is 19 × 14 cm., shows no signs of secondary wood. This late appearance of secondary xylem and other anatomical features 171suggest the possibility of the specific identity of L. Wünschianum and L. Harcourtii[364].

In 1871 Binney[365] described a specimen of a heterosporous cone, Lepidostrobus Wünschianus, from Arran exhibiting the ordinary features of lepidodendroid strobili; this was probably borne by Lepidodendron Wünschianum.

7. Lepidodendron macrophyllum (Williamson). Fig. 186, C.

The diagrammatic sketch reproduced in fig. 186, C, was made from the transverse section of a small twig, slightly less than 2 cm. in its longest diameter, originally figured by Williamson[366] in 1872. Earlier in the same year Carruthers[367] published a short account of the same form based on specimens collected by Mr Butterworth from the Coal-Measures of Lancashire near Oldham, but both authors refrained from instituting a new specific name. In a later publication Williamson spoke of the type as Lepidodendron macrophyllum[368]. Williamson’s species has nothing to do with Lycopodites macrophyllus of Goldenberg[369]. The most striking feature of this rare form is the large size of the leaf-cushions, which are of the Lepidophloios type, in proportion to the diameter of the shoot. The stele consists of a ring of xylem, all of which is primary in the sections so far described, enclosing a parenchymatous pith: a Stigmarian rootlet is shown at s.

8. Lepidodendron Veltheimianum Sternb. (General account). Figs. 157, 185, 186, A, B.

The above list may serve to call attention to a few synonyms[370] of this plant, and to a selection of sources from which full information may be obtained as to the history of our knowledge of this characteristic and widely spread Lower Carboniferous type.

Lepidodendron Veltheimianum is represented by casts of stems, the largest of which hitherto described reaches a length of 5·22 metres with a maximum diameter of 63 cm.; this specimen, figured by Stur[371], consists of a tapered main axis giving off smaller lateral shoots, some of which exhibit dichotomous branching. Fig. 185, C and D, represent the external features of a well-preserved cast and impression respectively. Oblique rows of prominent cushions wind round the surface of the stem and branches: each cushion is prolonged upwards and downwards in the form of a narrow ridge with sloping sides which connects adjacent cushions by an ogee curve. At the upper limit of the broader kite-shaped portion of the cushion the ligular pit forms a conspicuous feature; immediately below this is the leaf-scar with its three small scars,—the lateral parichnos strands and the central leaf-trace. The two oval areas shown in fig. 185, D, just below the lower edge of the leaf-scars, represent the parichnos arms which impinge on the surface of the cushions on their way to the leaves, as explained on a previous page. It is possible that these areas were visible on the living stem as strands of loose parenchyma comparable with the lenticel-like pits on the stipules of Angiopteris[372] and the leaf-bases of Cyatheaceous ferns, or it 173may be that their prominence in the specimen before us is the result of the decay of a thin layer of superficial cortex which hid174 them on the living tree. Fig. 185, B, illustrates the appearance of a stem in a partially decorticated condition (Bergeria state). A further degree of decortication is seen in fig. 185, A, which represents the Knorria condition.

Fig. 185. Lepidodendron Veltheimianum. From specimens in Dr Kidston’s Collection. (Approximately nat. size.)

Fig. 157 shows a Ulodendron axis of this species; in the lower part the specimen illustrates the partial obliteration of the surface features as the result of the splitting of the outer bark consequent on growth in thickness of the tree. By an extension of the cracks, shown in an early stage in fig. 157, the leaf-cushions would be entirely destroyed and the surface of the bark would be characterised by longitudinal fissures simulating the vertical grooves and ridges of a Sigillarian stem. The large stumps of trees shown in the frontispiece to Volume I. are probably, as Kidston[373] suggests, trunks of L. Veltheimianum in which the leaf-cushions have been replaced by irregular longitudinal fissures. In old stems of Sigillaria the enlarged parichnos areas constitute a characteristic feature (p. 205), but it does not follow that the absence of large parichnos scars is a distinguishing feature of all Lepidodendra.

In this species, as in others, the form of the leaf-cushion exhibits a considerable range of variation dependent on the thickness of the shoot; the contiguous cushions of young branches become stretched apart as the result of increasing girth of the whole organ, and casts of still older branches may exhibit very different surface-features[374]. The leaves as seen on impressions of slender branches are comparatively short, reaching a length of 1–2 cm. It is important to notice that leafy twigs of this species may bear terminal cones[375] resembling in form those of Picea excelsa and other recent conifers, though differing essentially in their morphological features.

The fossil stumps of trees represented in the frontispiece to Volume I. bear horizontally spreading and dichotomously branched root-like organs having the characters of Stigmaria ficoides[376]. Geinitz has suggested that Stigmaria inaequalis Göpp. may be the underground portion of Lepidodendron Veltheimianum.

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It is unfortunately seldom possible to connect petrified Lepidodendron cones with particular species of the genus based on purely vegetative characters, but it is practically certain that we are justified in recognising certain strobili described by Williamson[377] from the Calciferous Sandstone series of Burntisland on the Firth of Forth as those of Lepidodendron Veltheimianum. Williamson believed that the cone which he described belonged to the plant with shoots characterised by the anatomical features of his species Lepidodendron brevifolium (= L. Veltheimianum), a conclusion which is confirmed by Kidston[378]. The cone of L. Veltheimianum, which reached a diameter of at least 1 cm. and a length of 4 cm., agrees in essentials with other species of Lepidostrobus; the axis has a single medullated stele of the same general type as that of the vegetative shoots of Lepidodendron fuliginosum and L. Harcourtii. The sporophylls are described by Williamson as spirally disposed, and Scott notices that in some specimens they are arranged in alternate whorls; as in recent Lycopods both forms of phyllotaxis may occur in the same species. The heterosporous nature of this strobilus, to which Scott first applied the name Lepidostrobus Veltheimianus, is clearly demonstrated by the two longitudinal sections contributed by Mr Carruthers and figured by Williamson in 1893[379].

Each sporophyll, attached almost at right angles to the cone-axis, bears a radially elongated sporangium seated on the median line of its upper face; its margins are laterally expanded as a thin lamina; from the middle of the lower face a narrow keel extends downwards between two sporangia belonging to a lower series. From the base of a sporangium a mass of sterile tissue penetrates into the spore-producing region as in the large sporangia of Isoetes (cf. fig. 191, H, a, and fig. 133, H). The distal and free portion of the sporophylls is bent upwards as a protecting bract. Some of the sporangia in the upper part of the cone produced numerous microspores, while 8–16 176megaspores occur in the lower sporangia. The megaspores, having a mean diameter of 0·8 mm. “quite 40 times the size of the microspores[380],” are characterised by tubular capitate appendages, and by a conspicuous three-lobed projection (fig. 191, E)[381] which, as Scott suggests, may represent the outer spore-wall which has split as the result of germination. It is not improbable, as shown in fig. 191, I, that this cap was present before germination. The megaspores represented in fig. 191, I, illustrate their characteristic form as seen in a section of a megasporangium, Sm; the open beak-like portion of the larger spore is probably the apical region which has split along the three-rayed lines. These lines form a characteristic feature of 177both recent and extinct spores and denote their origin in tetrads. The spore shown in fig. 191, E[382], illustrates the external features. The apical region of the prothallus of a megaspore of Lepidodendron Veltheimianum described by Mr Gordon[383] consists of smaller cells than those occupying the greater part of the spore-cavity, a differentiation which he compares with that of the prothallus of Selaginella.

Fig. 186.

• A, B. Lepidodendron Veltheimianum. (Botany School, Cambridge.)
• C. Lepidodendron macrophyllum. (British Museum. No. 377.)
• x, Primary xylem; x², secondary xylem; s, Stigmarian rootlet.

There can be little doubt that the petrified shoots described by Williamson[384] from the Calciferous Sandstone beds of Burntisland as Lepidodendron brevifolium are identical with specimens possessing the external features of L. Veltheimianum. In 1872 Dawson expressed the opinion that Williamson’s species should be referred to L. Veltheimianum, and evidence subsequently obtained confirms this view. The stele of this species is of the medullated type, differing from that of L. fuliginosum and L. Harcourtii in the absence of prominent ridges on the external surface of the primary xylem, and from L. vasculare in the possession of a parenchymatous pith. In younger twigs the cortex consists of fairly homogeneous tissue, but in older branches there is a greater distinction between a delicate middle cortex and a stronger outer cortex. Fig. 186, A, represents a stem in which the vascular cylinder is composed of a primary xylem ring, x, 1·5 mm. broad, succeeded by a zone of secondary wood 1·2 cm. in breadth. The junction between the primary and secondary xylem is shown on a larger scale in fig. 186, B. The tissues abutting on the secondary xylem have not been preserved; the outer cortex, which consists chiefly of secondary elements, is divided superficially into unequal ridges corresponding to the leaf-cushions which have been more or less obliterated as the result of growth in thickness of the stem.

9. Lepidodendron Pedroanum (Carruthers).

In 1869 Mr Carruthers described some specimens of vegetative stems and isolated sporangia, collected by Mr Plant in Brazil, as Flemingites Pedroanus[385]. From a more recent account 178published by Zeiller[386] it is clear that Carruthers’ species is a true Lepidodendron; an examination of the type-specimens in the British Museum confirms this determination. The contiguous leaf-cushions have rounded angles similar in form to those of Lepidodendron Veltheimianum and L. dichotomum, but it is not unlikely that the Brazilian plant is specifically distinct from European species. A figure of one of the specimens on which Carruthers founded the species is given by Arber[387] in his Glossopteris Flora. The Brazilian plant is chiefly interesting as affording proof of the existence of Lepidodendron in the southern hemisphere; the species has also been recognised in South Africa from material collected by Mr Leslie at Vereeniging[388].

As Zeiller[389] has suggested, it is not improbable that the fossils described by Renault[390] from Brazil as Lycopodiopsis Derbyi may be the petrified stems of Lepidodendron Pedroanum. The structure of the central cylinder of Renault’s species is of the type represented by L. Harcourtii; the xylem forms a continuous ring and does not consist of separate strands of tracheae as Renault believed.

10. Lepidodendron australe (M’Coy). Figs. 187, A–C.

Specimens described under this name are interesting rather on account of their extended geographical range and geological antiquity than on botanical grounds. The drawings reproduced in fig. 187 illustrate the characteristic appearance of this Lower Carboniferous and Upper Devonian type, as represented by a specimen recently described[391] from the Lower Karroo (Dwyka) series, which is probably of Carboniferous age, near Orange River Station, South Africa. The surface is divided into polygonal or rhomboidal areas (figs. A and B) 8–9 mm. long and 7–8 mm. broad, arranged in regular series and representing leaf-scars, comparable with those of Sigillaria Brardi and other species, or possibly partially decorticated leaf-cushions. A short 179distance below the apex of each area there is a more or less circular prominence or depression (fig. 187, B) and on a few of the areas there are indications of a groove (fig. A, g) extending from the raised scar to the pointed base, as at g, g.

Fig. 187. Lepidodendron australe. Fig. A, nat. size.

In examining the graphitic layer on the surface of the South African specimen shown in fig. 187, A, use was made of a method recently described by Professor Nathorst[392]. A few drops of collodion were placed on the surface, and after a short interval the film was removed and mounted on a slide. The addition of a stain facilitated the microscopic examination and the drawing of the collodion film. The cell-outlines (fig. 187, C) on the surface of the polygonal areas may be those of the epidermis, but they were more probably formed by a subepidermal tissue; the scar, which interrupts the continuity of the flat surface, may mark the position of a leaf-base, or, 180assuming a partial decortication to have occurred prior to fossilisation, it may represent a gap in the cortical tissue caused by the decay of delicate tissue which surrounded the vascular bundle of each leaf in its course through the cortex of the stem. If the impression were that of the actual surface of a Lepidodendron or a Sigillaria, we should expect to find traces of the parichnos appearing on the leaf-scar as two small scars, one on each side of the leaf-bundle. In specimens from Vereeniging described in 1897[393] as Sigillaria Brardi, which bear a superficial resemblance to that shown in fig. A, the parichnos is clearly shown. On the other hand, an impression of a partially decorticated Lepidodendroid stem need not necessarily show the parichnos as a distinct feature: owing to its close association with the leaf-trace in the outer cortex, before its separation in the form of two diverging arms, it would not appear as a distinct gap apart from that representing the leaf-bundle. The absence of the parichnos may be regarded as a point in favour of the view that the impression is that of a partially decorticated stem. Similarly, the absence of any demarcation between a leaf-cushion and a true leaf-scar such as characterises the stems of Lepidodendra and many Sigillariae is also favourable to the same interpretation.

In 1872 Mr Carruthers[394] described some fossils from Queensland, some of which appear to be identical with that shown in fig. 187 under the name Lepidodendron nothum, Unger[395], a species founded on Upper Devonian specimens from Thuringia. The Queensland plant is probably identical with Dawson’s Canadian species, Leptophloeum rhombicum[396]. In 1874 M’Coy[397] instituted the name Lepidodendron australe for some Lower Carboniferous specimens from Victoria, Australia: these are in all probability identical with the Queensland fossils referred by Carruthers to Unger’s species, but as the identity of the German and Australian plants is very doubtful[398] it is better to adopt M’Coy’s specific designation.

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Krasser[399] has described a similar, but probably not specifically identical, type from China; from Devonian rocks of Spitzbergen Nathorst[400] has figured, under the name Bergeria, an example of this form of stem, and Szajnocha[401] has described other specimens from Lower Carboniferous strata in the Argentine.

Lepidodendron australe has been recorded from several Australian localities[402] from strata below those containing the genus Glossopteris and other members of the Glossopteris, or, as it has recently been re-christened, the Gangamopteris[403] Flora.

viii. Fertile shoots of Lepidodendron.

A. Lepidostrobus.

The generic name Lepidostrobus was first used by Brongniart[404] for the cones of Lepidodendron, the type-species of the genus being Lepidostrobus ornatus, the designation given by the author of the genus to a Lepidostrobus previously figured by Parkinson[405] in his Organic Remains of a Former World. The generic name Flemingites proposed by Carruthers[406] in 1865, under a misapprehension as to the nature of spores which he identified as sporangia, was applied to specimens of true Lepidostrobi. Brongniart also instituted the generic name Lepidophyllum for detached leaves of Lepidodendron, both vegetative and fertile; the specimen figured by him in 1822 as Filicites (Glossopteris) dubius[407], and which was afterwards made the type-species of the genus, was recognised as being a portion of the lanceolate limb of a large single-veined sporophyll belonging to a species of Lepidostrobus.

In an unusually large Lepidophyllum, or detached sporophyll of Lepidostrobus, in the Manchester University Museum, the free laminar portion reaches a length of 8 cm.

It is not uncommon to find Lepidodendron preserved in the form of a shell of outer cortex, which has become separated along the phellogen from the rest of the stem; as the result of 182compression the cylinder of bark may assume the appearance of a flattened stem covered with leaf-cushions. A specimen preserved in this way was described by E. Weiss as a cone of Lomatophloios macrolepidotus Gold., and is quoted by Solms-Laubach and other authors[408] as an example of an unusually large Lepidostrobus. An examination of the type-specimen in the Bergakademie of Berlin convinced me that Weiss had mistaken the partially destroyed leaf-cushions for sporophylls, and Stigmarian rootlets, which had invaded the empty space, for sporangia[409].

In external appearance some species of Lepidostrobus bear a superficial resemblance to the cone of a Spruce Fir (Picea excelsa), but the surface of a lycopodiaceous strobilus is usually covered by the overlapping and upturned laminae which terminate the more or less horizontal sporangium-bearing portion of the sporophyll.

Fig. 188 affords a good example of a long and narrow Lepidostrobus. This specimen from the Middle Coal-Measures of Lancashire has a length of 23 cm.; like other Lepidostrobi it is borne at the tip of a slender shoot. The fossil is sufficiently well preserved to show the characteristic radially elongated form of the large sporangia and the long and upturned distal portions of the sporophylls.

We may briefly describe Lepidostrobus as follows:—Cylindrical strobili consisting of an axis containing a single cylindrical stele which agrees generally with that of the vegetative shoots of L. Harcourtii and other species. The amount of parenchymatous pith varies in different forms; in some the primary xylem is almost solid. The middle cortical region, which has usually been destroyed before fossilisation, possesses the loose lacunar structure characteristic of this region in the vegetative branches. The thicker walled outer cortex is continued at the periphery into crowded, usually spirally disposed sporophylls, each of which consists of a more or less horizontal pedicel, which may be characterised by a keel-like median ridge on its lower surface, while to the central region of the upper face is attached a large radially elongated sporangium. One of the chief differences between a Lepidodendron cone 183and those of the recent genus Lycopodium is the greater radial elongation of the sporangia in the former. Some species of Lepidostrobus may have been homosporous; some are known to be heterosporous. In the latter the megasporangia borne on the lower sporophylls usually contain several megaspores as in184 Isoetes (cf. fig. 133, E). Beyond the distal end of the sporangium the sporophyll becomes broader in a horizontal plane and is bent upwards as a lanceolate limb; it may also be prolonged a short distance downwards as a bluntly triangular expansion.

Fig. 188. Lepidostrobus. Middle Coal-Measures, Bardsley, Lancashire. From a specimen in the Manchester Museum. (½ nat. size.)

185

There can be little doubt that the Palaeozoic Lepidodendra, like Lycopodium cernuum (fig. 123) and other recent Lycopods, usually bore their cones at the tips of slender shoots. The fertile shoot of Lepidophloios scoticus shown in fig. 160, B, affords one of several instances supporting this statement; similar examples are figured by Brongniart[410], Morris[411], and by more recent writers. The apparently sessile cone figured by Williamson[412] from a specimen in the Manchester Museum is certainly not in situ, but is accidentally associated with the stem.

The general absence of secondary wood in the steles of Lepidostrobi is, as Dr Kidston[413] points out, consistent with the view that the cones were shed on maturity and that fertilisation probably took place on the ground, or perhaps on the surface of the water where the slender hairs of the megaspores (fig. 191, F, I) may have served to catch the microspores.

Fig. 189. Lepidostrobus. Section through the apical region of a cone above the axis. (Manchester University Collection.)

Fig. 189 is an accurate representation of a transverse section, 6 mm. in diameter, of what is no doubt the apical portion of a Lepidostrobus from the Coal-Measures of Shore, Lancashire. The section cuts across the upturned free laminae above the level of the apex of the cone-axis. Each lamina contains a small vascular bundle composed of a few tracheae and some thin-walled cells surrounded by delicate mesophyll tissue. Immediately in front of the distal end of a sporangium a small ligule is borne on the upper face of the sporophyll (fig. 191, A, B, l) occupying the same position as in Selaginella (cf. fig. 131, F). Strands of vascular tissue pass in a steeply ascending course from the xylem to the pedicels of sporophylls, finally curving upwards and ending in the upper limb. Each vascular bundle consists of a strand of xylem, apparently of mesarch structure, accompanied by a few layers of parenchyma on its outer face and by a group of cambiform elements, the whole being enclosed in a sheath of parenchyma continuous with the inner cortex of the cone axis. The vascular bundle is accompanied by a parichnos in the outer cortex and in the sporophyll.

Reference has already been made to the belief on the part of some palaeobotanists that the large scars of Ulodendron represent attachment-surfaces of sessile cones, and reasons have been given against the acceptance of this view.

There is considerable range in the size of Lepidostrobi. An incomplete specimen, 33 cm. long and 6 cm. broad, which may have been 50 cm. in length, is described by Renault and Zeiller[414] from the Commentry Coal-field. The larger cones afford a striking demonstration of the enormous spore-output of some species of Lepidodendron.

Among the earliest accounts of the anatomy of Lepidostrobus are those by Hooker[415] and Binney[416]. One of the specimens 186described by the former author (fig. 190) affords an interesting example of an unusual manner of fossilisation; a hollow stem or Lepidodendron is filled with sedimentary material containing several pieces of Lepidostrobi in an approximately vertical position.

Fig. 190. Lepidodendron stem with Lepidostrobi. (After Hooker.)

1. Side-view showing leaf-cushions on the left-hand side and the Knorria condition on the right.
2. View of transverse section; s, sections of Lepidostrobi.

The fact that Lepidostrobi usually occur as isolated specimens renders it impossible in most cases to refer them to particular species of Lepidodendron. Neither external features nor anatomical characters afford satisfactory criteria by which to correlate vegetative and fertile shoots; in some measure this is due to the imperfection of our knowledge as regards the range of structure within the limits of species; it is also due187 to lack of information as to the extent to which the transition from sterile to fertile portions of a shoot is accompanied by anatomical differences. Prof. Williamson wrote: “I have for many years endeavoured to discover some specific characters by which different Lepidostrobi can be distinguished and identified, but thus far my efforts have been unsuccessful[417].” In a few cases, such as those mentioned in the description of Lepidodendron Veltheimianum and L. Wünschianum, it has been possible to correlate cones and vegetative shoots.

The most complete account we possess of the anatomy of Lepidodendron cones is that by Mr Maslen[418], who first demonstrated the occurrence of a ligule on the sporophylls, and thus supplied a missing piece of evidence in support of the generally accepted view as to the homology of the sporangium-bearing members and foliage leaves.

i. Lepidostrobus variabilis (Lindley and Hutton).

Under this specific name are included strobili from Upper Carboniferous rocks which, in spite of minor differences, may be considered as one type. The cylindrical cones vary considerably in size, some reaching a length of 50 cm. or more. The sporophylls are attached by a pedicel, 4–8 mm. long, at right angles to the axis, while the distal portion forms an oval lanceolate limb 10–20 mm. in length. The sporangia are 4–8 mm. long.

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The branched example figured by Lindley and Hutton[419] as a variety (L. ornatus var. didymus) illustrates a phenomenon not uncommon in both Palaeozoic and recent lycopodiaceous strobili.

Fig. 191. Lepidostrobus.

• A–D. L. oldhamius.
• B, C, D. From sections in the Binney Collection, Cambridge.
• E. Megaspore. (After Kidston.)
• F. Megaspore (Coal-Measures, Halifax). (After Williamson.)
• G. Megaspore of Lepidostrobus foliaceus. (After Mrs Scott.)
• H. Tangential section of sporangium. (After Bower.)
• I. Part of sporangium wall, Sm, of the cone of Lepidodendron Veltheimianum, enclosing two megaspores. (Cambridge Botany School.)

ii. Lepidostrobus oldhamius Williamson[420]. Fig. 191, A–D.

Williamson[421] instituted this term for strobili previously described by Binney[422], without adequate evidence, as the cones 189of Lepidodendron Harcourtii. In shape and in the main morphological features this type resembles L. variabilis, which is however known only in the form of casts and impressions. A cone of L. oldhamius, 2–3 cm. in diameter, possesses a medullated stele consisting of a ring of primary xylem (fig. 191, D, x) with exarch protoxylem and no secondary elements. Maslen found several short tracheae at the periphery of the xylem and states that these led him to compare the cone with the vegetative shoots of Lepidodendron vasculare, but the common occurrence of such elements in different types of shoot renders them of little or no specific value. The inner cortex is like that of vegetative shoots of Lepidodendron and the middle cortex, which was no doubt of the type described in Lepidostrobus Brownii, is represented by a gap in the sections, beyond which is the stronger outer cortex (fig. 191, D) passing into the horizontal pedicels of the sporophylls. The section of the axis reproduced in fig. 191, D, was figured by Binney[423] as Lepidodendron vasculare. The leaf-traces, several of which are seen in the middle cortical region in fig. D, lt, consist of a strand of scalariform tracheae, with a mesarch protoxylem, succeeded by a few parenchymatous cells; beyond these there is usually a small gap which was originally occupied by a strand of thin-walled cells. It is important to note that in one sporophyll-trace figured by Maslen[424] there is a strand of thin-walled elongated elements abutting on the xylem, which he describes as phloem. This tissue is certainly more like true phloem than any which has hitherto been described in the leaf-traces of vegetative shoots. The state of preservation is not, however, sufficiently good to enable us to recognise undoubted phloem features.

In such cones as I have examined no tissue has been seen which shows the histological features characteristic of the secretory zone of vegetative shoots: the “phloem” (Maslen) occupies the position in the sporophyll bundle which in the vascular bundles of foliage leaves is occupied by a dark-celled and partially disorganised tissue in continuity with the secretory zone of the main stele. It may be that in the strobili 190this tissue occurred in a modified form, but even assuming that the section figured by Maslen shows true phloem, an assumption based on slender evidence, this is not sufficient justification for the application of the term phloem to a tissue occupying a corresponding position in vegetative shoots and distinguished by well-marked histological features.

The sporophyll-traces, as seen in the outer cortex in fig. 191, D, are partially surrounded by a large crescentic space, p, which was originally occupied by the parichnos. The sporangia are attached along the middle line of the sporophyll and, as in Lepidostrobus Brownii, a cushion of parenchyma projects into the lower part of the sporangial cavity (fig. 191, A, a; C, a).

The diagrammatic sketch of part of a section in the Binney Collection reproduced in fig. 191, B, shows the position of the ligule, l. No megaspores have been discovered in any specimens of this type; the microspores, which occur both singly and in tetrads, have a length of 0·02–0·03 mm.

The drawing shown in fig. 191, A, based on a section in the Binney Collection, illustrates the general arrangement of the parts of a typical Lepidostrobus. I have made use of this sketch instead of that given by Maslen, as his figure conveys the idea that the sporophylls are superposed, whereas, whether they are verticillate or spiral, a radial longitudinal section would not cut successive sporangia in the same plane.

iii. Lepidostrobus Brownii (Brongn.).

In 1843 a specimen of a portion of a petrified cone was purchased by the British Museum, assisted by the Marquis of Northampton and Robert Brown, for £30 from a French dealer. This fossil, from an unknown locality, was briefly described by Brown in 1851[425] and named by him Triplosporites, but in a note added to his paper he expressed the opinion that the generic designation Lepidostrobus would be more appropriate. Brongniart afterwards named the cone Triplosporites Brownii[426], and Schimper[427] described it in his Traité as Lepidostrobus191 Brownii. The type-specimen is preserved in the British Museum and the Paris Museum possesses a piece of the same fossil.

The central axis of the cone has a stele of the type characteristic of Lepidodendron fuliginosum and L. Harcourtii, and the xylem is surrounded by a thin-walled tissue described by Bower[428] as possibly phloem; but in the absence of longitudinal sections it is impossible to say how far the tissue external to the xylem agrees with that in Lepidodendron stems. The sporophylls consist of a horizontal portion, to the upper face of which the radially elongated sporangia are attached, one to each sporophyll; beyond the distal end of the sporangium the sporophyll bends sharply upwards as a fairly stout lamina. The wall of the sporangium is composed of several layers of cells, as shown in a drawing published by Bower[429]; in the interior occur groups of microspores, and from a ridge of tissue which extends along the whole length of the sporangium irregular trabeculae of sterile tissue project into the sporangial cavity, as in Isoetes (fig. 191, H: cf. fig. 133, H).

Further information in regard to Lepidostrobus Brownii has recently been supplied by Prof. Zeiller[430], who recognises the existence of a ligule, and draws attention to some interesting histological features in the tissue of the sporophylls[431].

Spores of Palaeozoic Lycopodiales.

The calcareous nodules from the Coal seams of Yorkshire and Lancashire are rich in isolated spores, many of which are undoubtedly those of Lepidostrobi. Examples of spores were figured by Morris[432] in 1840, and their occurrence in coal has been described by several authors, one of the earliest accounts being by Balfour[433]. The drawings of Palaeozoic and recent spores published by Kidston and Bennie[434] demonstrate a striking similarity between the megaspores of existing and extinct Lycopods, the chief difference being the larger size of the fossils.

192

The general generic name Triletes, originally used by Reinsch[435], is a convenient term by which to designate Pteridophytic spores which cannot be referred to definite types.

It is usual to find more than four megaspores in each megasporangium in Palaeozoic and not infrequently, as we have seen, in Mesozoic lycopodiaceous strobili, but in some Palaeozoic cones, e.g. Bothrostrobus (fig. 216) and Lepidostrobus foliaceus[436], a single tetrad only appears to have reached maturity.

The occurrence of long simple or branched and sometimes capitate hairs is a common feature of Carboniferous megaspores (fig. 191, E, F, I). It is possible that these appendages served to catch the microspores, thus facilitating fertilisation. A peculiar form of megaspore has been described by Mrs Scott[437], and assigned by her to Lepidostrobus foliaceus, the megasporangium of which apparently contained only four spores. As shown in fig. 191, G, a large bladder-like appendage characterised by radiating veins is attached to the thick spore-coat; it is suggested that this excrescence may be compared with the “swimming” apparatus of the recent water-fern Azolla. The epithet swimming which it is customary to apply to the appendages of Azolla megaspores would seem to be inappropriate if Campbell[438] is correct in stating that spores of Azolla are incapable of floating.

B. Spencerites.

Spencerites insignis (Williamson). Fig. 192.

Another type of lycopodiaceous strobilus, differing sufficiently from Lepidostrobus to deserve a special generic designation, 193is that originally described by Williamson[439], from the Lower Coal-Measures of Yorkshire, as a type of Lepidostrobus, L. insignis, but afterwards[440] more fully investigated and assigned to a new genus by Scott[441]. It should be pointed out that in a later publication Williamson spoke of the lycopodiaceous axis, which he suspected might belong to his L. insignis, as possibly worthy of recognition as a distinct generic type.

Fig. 192. Spencerites insignis (Williamson). (After Miss Berridge.)

Of the two species included by Scott in his genus Spencerites only one, S. insignis, need be considered. Since the publication194 of Scott’s paper our knowledge of this type has been extended by Miss Berridge[442] and by Prof. Lang[443].

The axis of the strobilus has a stele characterised by a pith of elongated elements, most of which have thin walls; the xylem cylinder possesses about twenty protoxylem strands forming more or less prominent exarch ridges. The cortex exhibits a differentiation comparable with that in the shoots of Lepidodendron. The sporophylls are arranged in alternating verticils, each whorl consisting of ten members: the narrow horizontal pedicel of a sporophyll, containing a single vascular bundle, as shown in fig. 192, is expanded distally into a prominent upper lobe bearing a cushion of small and delicate cells, to which the sporangium is attached, and prolonged obliquely upwards as a free leaf-like lamina. The lower blunt prolongation of the sporophylls appears to form a thick dorsal lobe, but, as Lang has pointed out, it is highly probable that the present form of the dorsal lobe is of secondary origin, and is “due to the disappearance of a mucilage cavity from a large sporophyll base[444].” As Miss Berridge remarks, the vascular bundle of the sporophyll does not give off a branch to the ventral lobe and sporangium. In attachment, in shape, and in the structure of the wall the sporangia differ markedly from those of Lepidostrobi. The spores, which also constitute a characteristic feature of the genus, have a maximum diameter of 0·14 mm.; they are described as oblate spheroids with a broad hollow wing running round the equator (fig. 192) comparable with the air-sacs of the pollen of Pinus. Scott points out that the spores of Spencerites are intermediate in size between the microspores of Lepidodendron and the megaspores of Lycopodium; it is difficult therefore to decide to which category they should be referred. Spencerites is clearly distinct from Lepidostrobus; the absence of a ligule, the manner of attachment of the sporangia, and the form and size of the spores, are characteristic features.

A comparison of Spencerites with the strobili of Lycopodium cernuum (figs. 123, 126–129) has recently been made by Lang, who draws attention to the striking agreement as regards general plan and even detailed structural features between the Palaeozoic 195and the recent type of strobilus. It is interesting to find, as Lang points out, that in the original account of the fossil cone by Williamson, the view is expressed that the sporangiophores were confluent. An examination of the section figured by Williamson[445] led Lang to confirm this opinion. It would be out of place to enter here into a detailed comparison of Spencerites insignis and the cone of Lycopodium, but the resemblances are considered by Lang to be sufficiently close to suggest that the striking similarity may be indicative of relationship[446].

It is worthy of notice that the radial section of Spencerites (fig. 192) presents a fairly close resemblance to a corresponding section through a cone-scale of Agathis (Kauri Pine)[447]. In each case the megasporangium is attached by a narrow pedicel to the sporophyll and the latter has a similar form in the two plants, though the extent of the resemblance is somewhat lessened by Lang’s more complete account of the Palaeozoic type. If the Spencerites sporangia possessed an integument the similarity with the Agathis ovule would of course be much closer: recent palaeobotanical investigations have shown that ovules and sporangia are not separated by impassable barriers.

[Since this Chapter was set up in type a paper has appeared by Dr Bruno Kubart on a new species of Spencerites spore, S. membranaceus, from the Ostrau-Karwiner Coal-basin (Austria). The spores are larger than those of S. insignis and in some the cells of a prothallus are preserved. Kubart figures a section of a spore containing a group of seven cells, a central cell, which he regards as an antheridial mother-cell, surrounded by six wall-cells. Kubart (90).]

i. General.

In view of the close resemblance between Lepidodendron and Sigillaria, another lycopodiaceous plant characteristic of Carboniferous and Permian floras, a comparatively brief description of the latter genus must suffice, more particularly as Lepidodendron has received rather an undue share of attention. Sigillaria, though abundantly represented among the relics of Palaeozoic floras, especially those preserved in the Coal-Measures, is rare in a petrified state, and our knowledge of its anatomy is far from complete. In external form as in internal structure the difference between the two genera are not such as enable us to draw in all cases a clearly defined line of separation.

In the Antediluvian Phytology, Artis[448] figured a fossil from the Carboniferous sandstones of Yorkshire which he called Euphorbites vulgaris on account of a superficial resemblance to the stems of existing succulent Euphorbias. Rhode[449] also compared Sigillarian stems with those of recent Cacti. The specimen described by Artis is characterised by regular vertical and slightly convex ribs bearing rows of leaf-scars in spiral series, like those on the cushions of Lepidodendron. A few years earlier Brongniart[450] had instituted the genus Sigillaria[451] for plants with ribbed but not jointed stems bearing “disc-like impressions” (leaf-scars) disposed in quincunx; the type-species named by the author of the genus Sigillaria scutellata is identical, as Kidston[452] points 198out, with Euphorbites vulgaris of Artis and with the plant afterwards figured by Brongniart as S. pachyderma[453]. Brongniart in 1822 figured another type of stem characterised by the absence of ribs and by prominent spirally arranged cushions bearing relatively large leaf-scars like the upper part of the specimen shown in fig. 203; this he named Clathraria Brardii, a well-known and widely distributed Carboniferous and Permian species now spoken of as Sigillaria Brardi (figs. 196, A–C; 203). A third type of stem figured by Brongniart as Syringodendron striatum[454] agrees with Sigillaria scutellata in having ribs, but differs in the substitution of narrow oval ridges or depressions for leaf-scars; this is now recognised as a partially decorticated Sigillaria, in which the vascular bundle of each leaf is represented by a narrow ridge or depression. The name Syringodendron, originally used by Sternberg, is conveniently applied to certain forms of Sigillarian stems which have lost their superficial tissues. A fourth generic name, Favularia, was instituted by Sternberg[455] for Sigillarian stems with ribs covered with contiguous leaf-scars of hexagonal form and prominent lateral angles (fig. 193, A; fig. 200, G).

Fig. 193.

• A.  Sigillaria elegans Brongn.
• B.  Sigillaria rugosa Brongn. Middle Coal-Measures.
• C.  Omphalophloios anglicus Kidst. Barnsley.
• D.  Sigillaria elegans Brongn.
• E.  Sigillaria tessellata Brongn.
• (A, B, C, E, about ¾ nat. size. Dr Kidston’s Collection.)

The generic or sub-generic title Rhytidolepis, also instituted by Sternberg, is applied to ribbed Sigillarian stems such as S. scutellata, S. rugosa (fig. 193, B), S. mammillaris (fig. 195), or S. laevigata (fig. 196, D). Goldenberg[456] proposed the name Leiodermaria for smooth Sigillarian stems with leaf-scars not in contact with one another (fig. 196, C).

The shoot system of Sigillaria consisted of a stout stem tapering upwards to a height of 100 feet[457] or more as an unbranched column, with its dome-shaped apex[458] covered with linear grass-like leaves or, in some species, such as Sigillaria Brardi[459], S. Eugenii[460], etc., the main trunk was occasionally divided by apparently equal dichotomy. The younger portions of the stem or branches were in some species clothed with leaves separated by a narrow zigzag groove surrounding their hexagonal199 bases, while in other forms each leaf was seated on a more or less prominent cushion having the form illustrated by Sigillaria McMurtriei (fig. 194) or by the example represented in fig. 200, H; or as in the ribbed species shown in figs. 193, B, and 195, the leaves in vertical series were separated from one another by longer portions of the ribs. As in Lepidodendron the cushions are frequently characterised by irregular transverse wrinklings and other[461] surface-ornamentation which in some instances at least may have been produced as the result of post-mortem shrinkage of superficial tissue. From the rarity of shoots with the foliage attached, it would seem that the leaves persisted for a comparatively short time and were cut off by an absciss-layer leaving behind a well-marked leaf-scar area. The linear leaves, 200reaching in rare cases a length of one metre (e.g. S. lepidodendrifolia) but usually much shorter, possessed a single median bundle, and the lower face was characterised by two stomatal grooves and a median keel. It is not uncommon to find leaf-bases of Sigillaria detached from the stem and preserved as separate impressions. The term Sigillariophyllum used by Grand’Eury[462] may be applied to detached leaves, though it is by no means easy to distinguish between the foliage of Sigillaria and Lepidodendron. A comparison of a typical species of Sigillaria, such as S. rugosa (fig. 193, B) or S. Brardi (fig. 196, A–C) with a typical Lepidodendron reveals obvious differences in the form of the leaf-cushion, but in some cases the distinction becomes purely arbitrary.

Fig. 194. Sigillaria McMurtriei Kidst. From a specimen from the Upper Coal-Measures of Radstock, in the British Museum (V. 952). Nat. size.

Fig. 195. Sigillaria mammillaris. (Rhytidolepis form.) From a specimen in the Manchester Museum. p, parichnos; l, ligule-pit; t, leaf-trace; c, cushion; s, leaf-scar.

Fig. 196.

• A–C.  Sigillaria Brardi. (A after Germar; B, C after Zeiller.)
• D.  Sigillaria laevigata.
• E.  Lepidodendron Wortheni (D and E after Zeiller).

201

Immediately above the centre of the upper boundary of a Sigillarian leaf-scar a ligule pit may often be detected, as shown in fig. 195, l, and in some cases, e.g. a specimen figured by Germar[463] (fig. 196, A) as Sigillaria spinulosa (identical with S. Brardi), some circular scars with a central pit surrounded by a raised rim occur on the surface of the stem, either singly or in pairs, near the leaf-scars; these, it is suggested, may represent the position of adventitious roots or, as Germar thought, of some deciduous spinous processes. The leaf-scars are frequently hexagonal in shape, with the lateral angles either rounded (fig. 200, F) or sharply pointed (fig. 200, G, H); each scar bears three smaller scars as in Lepidodendron, a central circular, oval or crescentic leaf-trace scar and larger oval or slightly curved scars formed by the two parichnos arms (fig. 195, p). The larger size of the parichnos arms, the individual cells of which may often be detected as a fine punctation, is a distinguishing feature of the genus, but otherwise the structure is very similar to that in Lepidodendron. As shown in figs. 195, 200, F, G, the three scars may occur nearer the upper than the lower margin of the leaf-base area.

Lepidodendron Wortheni[464] (fig. 196, E), described from North America by Lesquereux[465], by Zeiller[466] from France, and by Kidston[467] from the Upper and Middle Coal-Measures of England, may be quoted as a Lepidodendron bearing a close resemblance to Sigillaria. The shoots bear cushions two or three times as long as broad and without the usual median division, but with numerous irregular and discontinuous transverse wrinklings. Lepidodendron Peachii Kidston[468] affords another example of a form agreeing both with Sigillaria and with Lepidodendron. An Upper Devonian type described by White[469] as Archaeosigillaria primaeva affords a striking instance of the combination on one stem of Sigillarian and Lepidodendroid leaf-cushions.

202

The difference between the original surface of a Sigillaria stem and that of partially decorticated specimens is seen in figs. 196, C and D; in fig. C the bark of Sigillaria Brardi shows the characteristic wrinklings of the superficial tissue, while at a slightly lower level the leaf-scars are replaced by the parichnos casts, a, and fine longitudinal striations represent the elongated phelloderm cells laid bare by the exfoliation of the surface-layers. Similarly, in the rib of Sigillaria laevigata (fig. 196, D) the parichnos arms, p, and the longitudinal striations are exposed at the lower level, while the surface is smooth and bears rows of widely separated leaf-scars.

Fig. 197. Carica sp. From the Royal Gardens, Kew. (Much reduced.) M.S.

203

The older part of a Sigillarian stem may present an appearance very different from that of the younger shoots. The leaf-cushions may be stretched apart as the result of elongation and increase in girth, while in some cases the arrangement of the leaf-scars may vary on the same axis as the result of inequalities in growth or changing climatic conditions. The contiguous arrangement of the leaf-scars and narrow cushions characteristic of the Clathrarian form of stem, as was first demonstrated by Weiss[470], and afterwards illustrated by Zeiller[471] and Kidston, may be gradually replaced (on the same specimen) by a more distant disposition of the leaf-scars separated by a smooth intervening surface of bark. The specimen of S. Brardi reproduced in part in fig. 203, and first figured by Kidston, affords an example of three “species” on one piece of stem, S. Brardi Brongn., S. denudata Goepp. and S. rhomboidea Brongn.[472]

The piece of Carica stem, represented in fig. 197, illustrates the danger of trusting to the disposition of leaves as a specific criterion.

Similarly, in the ribbed forms the degree of separation of the leaf-scars is by no means uniform in a single species[473]. Some authors have adopted a two-fold classification of Sigillarian stems proposed by the late Prof. Weiss[474] of Berlin, who divided the Sigillariae into (A) Sub-Sigillariae, comprising Leiodermariae and Cancellatae, and (B) Eu-Sigillariae, including Favulariae and Rhytidolepis. Grand’Eury[475] adopts the terms Rhytidolepis and Leiodermaria for ribbed and smooth stems respectively, the type to which the name Clathraria was applied by Brongniart being in some cases at least the young form of Leiodermarian stems. While recognising the artificial distinction implied by such terms as Rhytidolepis, Leiodermaria, and other sub-generic titles, we may conveniently speak of the two main types of Sigillaria stems as ribbed and smooth.

Still older stems of Sigillaria are not uncommon from which the leaf-scars and other superficial tissues have been exfoliated, leaving exposed a longitudinally fissured surface of secondary cortex characterised by pairs of considerably enlarged parichnos 204strands (fig. 198) which are sometimes partially or wholly fused into one (Syringodendron state of Sigillaria). The single or double nature of the elliptical or circular parichnos areas is doubtless due to the degree of exfoliation, which may extend sufficiently deep into the cortex to reach the level of the parichnos before the single strand has bifurcated (cf. Lepidodendron, p. 100). In the Museums of Manchester, Newcastle, and other places casts of large Sigillaria stems may be seen, which illustrate the differences in breadth and regularity of the vertical ribs, and in the size and shape of the parichnos areas in different regions of a partially decorticated stem. A cast of a ribbed species in the Manchester Museum, having a length of 185 cm. and a breadth of 56 cm., shows in the upper portion straight vertical grooves and broad ribs bearing pairs of parichnos scars 11 mm. long; in the lower portion the ribs tend to become obliterated and the parichnos scars, 2 cm. in length, may be partially fused and arranged in much less regular vertical series. A feature of these older ribbed Sigillarian stems is the increase in the number of the ribs from below upwards. Kidston[476] has described a specimen in the Sunderland Museum, 6 feet 6 inches long, with a circumference at the slightly bottle-shaped base of 5 feet. On the lower portion of the stem there are 29 broad ribs; about one-third the height many of these bifurcate, producing as many as 40 ribs in the upper part where the cast has a circumference of 3 feet. The increase in number of the ribs is due in part to bifurcation, but also to the intercalation of new ones. As Kidston points out, this example shows that as a stem grew in length additional leaves were developed at the apex. A similar stem, which illustrates very clearly the increase in the number of ribs from below upwards, may be seen in the Newcastle Museum.

Grand’Eury[477] has described an example of an old stem of a ribless species of Sigillaria, Syringodendron bioculatum, bearing single and double parichnos areas of nearly circular form and with a diameter of 1–2 cm. In a specimen figured by Renault and Roche[478] (Syringodendron esnostense) from the Culm strata 205in France, the parichnos scars reach a length of 3 cm. As seen in the fragment of a ribbed Sigillaria represented in fig. 198, the large parichnos areas exhibit a distinct surface pitting in contrast to the fine longitudinal striation of the rib; the difference in surface-appearance is due to the nature of the tissue, which in the parichnos consists of fairly large parenchymatous elements with groups of secretory cells[479], and in the exposed cortex of elongated elements. The vertical line in the middle of fig. 198, which occurs in the middle of the rib, has probably been formed by splitting of the bark.

Fig. 198. Sigillaria with large parichnos areas. (⅓ nat. size.) M.S.

Grand’Eury’s description of fossil forests of Sigillariae in the rocks of the St Étienne[480] district affords a striking picture of these arborescent Pteridophytes; he speaks of the stems of some of the trees as swollen like a bottle at the base, characterised by the Syringodendron features and terminating below in short repeatedly forked roots of the type known as Stigmariopsis. Other specimens of Sigillaria stumps show a marked decrease in girth towards the base; this tapered form is regarded by Grand’Eury as the result of the development of aerial columnar stems from underground rhizomes.

The nature of the root-like organs of Sigillaria is dealt with in the sequel: a brief reference may, however, be made 206to the occurrence of stumps of vertical trunks which pass downwards into regularly forked and spreading arms. These arms lie almost horizontally in the sand or mud like the underground rhizomes of Phragmites and other recent plants growing in swampy situations where water is abundant and where deeper207 penetration of the soil would expose them to an insufficient supply of oxygen[481]. It is certain that Sigillaria had no tap-root, but was supported on spreading subterranean organs bearing spirally disposed long and slender rootlets which absorbed water from a swampy soil.

Fig. 199. Partially decorticated stem of Sigillaria showing two zones of cone-scars. From a cast in the Sedgwick Museum, Cambridge. M.S. (⅕ nat. size.)

The regularity of the leaf-scar series on a Sigillarian stem may be interrupted by the occurrence of oval scars with a central scar and surrounding groove (fig. 193, E); these occur in zones at more or less regular intervals on the stem, as seen in the partially decorticated cast represented in fig. 199. Zeiller has pointed out that the rows of oval or circular scars, which mark the position of caducous stalked strobili, may occur between the leaf-scars in vertical series, each of which may include as many as 20 scars, while in other cases a single series of such cone-scars may encircle the stem[482]. The zones are usually of uneven breadth, as in S. Brardi, and their occurrence produces some deformation of the adjacent leaf-scars.

By the earlier writers Sigillaria was compared with succulent Euphorbias, Cacti, and Palms; Brongniart[483] at first included undoubted Sigillarian stems among Ferns, but after investigating an agatized stem from Autun, he referred Sigillaria to the Gymnosperms[484] on the ground that it had the power of producing secondary wood. It was then supposed that Lepidodendron possessed only primary xylem, and that the presence of a vascular meristem in Sigillaria necessitated its separation from the lycopodiaceous genus Lepidodendron and its inclusion in the higher plants. By slow degrees it was recognised, as in the parallel case of the genus Calamites, that the presence or absence of secondary vascular tissue is a character of small importance. Williamson, whose anatomical researches played the most important part in ridding the minds of palaeobotanists of the superstition that secondary growth in thickness is a monopoly of the Phanerogams, spoke in 1883 of the conflict as to the affinities of Lepidodendron and Sigillaria as virtually over but leaving here and there “the ground-swell of a 208stormy past[485].” In 1872 the same author had written: “If then I am correct in thus bringing the Lepidodendra and Sigillariae into such close affinity, there is an end of M. Brongniart’s theory, that the latter were gymnospermous exogens, because the cryptogamic character of the former is disputed by no one; we must rather conclude as I have done that the entire series represents, along with the Calamites, an exogenous group of Cryptogams in which the woody zone separated a medullary from a cortical portion[486].”

In 1879 Renault[487] expressed the opinion that Brongniart by his investigation of the anatomy of Sigillaria elegans had established in a manner “presque irréfutable” that Sigillaria must be classed as a Gymnosperm showing affinity with the Cycads.

In 1855 Goldenberg[488] described some strobili which he regarded as those of Sigillaria and recognised their close resemblance to a fertile plant of Isoetes. He was led to the conclusion, which had little influence on contemporary opinion, that Sigillaria is related to Isoetes and must be classed among Pteridophytes. To these long and narrow strobili Schimper gave the name Sigillariostrobus[489]. In 1884 Zeiller[490] supplied confirmation of Goldenberg’s view by the discovery of cones borne on pedicels with Sigillarian leaf-scars, thus demonstrating the generic identity of cones and vegetative shoots, which Goldenberg had connected on the evidence of association. Zeiller’s more recent work[491] and the still later researches of Kidston[492] have added considerably to our knowledge of the morphology of Sigillarian cones. Grand’Eury’s remark made so recently as 1890[493] that opinion in regard to the Gymnospermous nature of Sigillaria is losing ground every day, bears striking testimony to the pertinacity with which old beliefs linger even in the face of overwhelming proof of their falsity.

It is remarkable, in view of the abundance of vegetative shoots, how rarely undoubted Sigillarian strobili have been 209found; this may, however, be in part due to a confusion with Lepidostrobi which so far as we know do not differ in important respects from Sigillariostrobi[494].

There can be no doubt that Sigillaria usually produced its cones on slender pedicels which bore a few leaves or bracts in irregular verticils, or in short vertical series on comparatively stout stems, an arrangement reminding us of the occurrence of flowers on old stems of Theobroma and other recent Dicotyledons. As Renault[495] pointed out the fertile shoots are axillary in origin.

Dr Kidston[496] is of opinion that certain species of Sigillaria bore cones sessile on large vegetative shoots characterised by two opposite rows of cup-like depressions like those in the Ulodendron form of Lepidodendron Veltheimianum (fig. 157). He has described the Ulodendron condition of two species, Sigillaria discophora (König) and S. Taylori (Carr.); the cup-like depressions may have a diameter of several centimetres and are distinguished from those of Bothrodendron by the almost central position of the umbilicus. The specimens which he figures as S. discophora are identified by him with the stem figured by König as Lepidodendron discophorum and by Lindley and Hutton[497] as Ulodendron minus. We have already dealt with the nature of Ulodendron shoots, expressing the opinion that in spite of the often quoted specimen described by D’Arcy Thompson[498], in which a supposed cone occurs in one of the cups, there is no satisfactory case of any undoubted cone having been found attached to the large Ulodendron scars. It is more probable that the Ulodendron depressions represent the scars of branches, either elongated axes, or possibly in some cases deciduous tuberous shoots which served as organs of vegetative reproduction. A specimen figured by Kidston as Sigillaria Taylori from the Calciferous sandstone of Scotland[499] bears a row of slightly projecting “appendicular organs” attached to a Ulodendron axis; but these furnish no proof of their strobiloid nature. The main question is, are these Ulodendron shoots correctly identified by Kidston as Sigillarian? 210The surface of the specimens shows crowded rhomboidal scars surrounded in some cases by a very narrow border or cushion; the general appearance is, as Kidston maintains, like that of Sigillaria Brardi in which the leaf-scars are contiguous (e.g. fig. 203, upper part). None of the leaf-scars exhibit the three characteristic features, the leaf-trace and parichnos scars, but only one small scar appears on each leaf-base area. In a more recent paper Kidston figures a small piece of a stem from Kilmarnock, which he identifies as Sigillaria discophora, showing the three characteristic scars on the leaf-base area. There is no doubt as to the Sigillarian nature of this specimen, but it is not clear if the piece figured is part of a Ulodendron shoot[500].

Prof. Zeiller[501] retains the older name Ulodendron minus Lind. and Hutt. in place of König’s specific designation and dissents from Kidston’s identification of Ulodendron minus and U. majus of Lindley and Hutton as one species; he is also inclined to refer these Ulodendron axes to Lepidodendron. In spite of the superficial resemblance to Sigillaria of the specimens described by Kidston, and which I have had an opportunity of examining, I venture to regard their reference to that genus as by no means definitely established. We must recognise the difficulty in certain cases of drawing any satisfactory distinction between Sigillaria and Lepidodendron based on external features, and while giving due weight to the conclusions of so experienced a palaeobotanist as my friend Dr Kidston, I venture to think we are not in a position to state with confidence that Sigillaria possessed Ulodendron shoots.

ii. Leaves.

The leaves of Sigillaria agree closely with those of Lepidodendron; they are either acicular (fig. 200, D) like Pine needles or broader and flatter like the leaves of Podocarpus. Their attachment to comparatively thick branches[502] 211shows that they persisted, in some cases at least, for several years as in Araucaria imbricata. The lower surface of the lamina was characterised by a prominent keel (fig. 142, A and C) which dies out towards the apex; on either side of it are well-defined stomatal grooves (figs. 142, g, g; 143, A; 200, D, g). The upper face may be characterised by another groove (fig. 142, B) but without stomata. The occurrence of the stomatal grooves, the abundance of transfusion tracheae (fig. 142, t) surrounding the vascular bundle, and the presence of strengthening hypodermal tissue suggest that the leaves of Sigillaria were of a more or less pronounced xerophilous type and had a fairly strong and leathery lamina. The mesophyll tissue consists either of short parenchymatous cells or of radially elongated palisade-like elements and has the loose or lacunar arrangement characteristic of the aerating system in recent leaves; the slight development or absence of palisade-tissue may indicate exposure to diffuse light of no great intensity.

In most species there is a single vein, but in others the xylem forms a double strand (fig. 142, B). Sections of the lamina near the apical region present a more circular form, owing to the gradual obliteration of the upper groove and lower keel and to the dying out of the stomatal grooves.

The transverse section of the leaf diagrammatically represented in fig. 142, A, A′, shows the two stomatal grooves, g, and a prominent keel; the single vein consists of a small group of primary tracheae, x, some delicate parenchyma, and a brown patch of imperfectly preserved tissues, a, resembling the secretory zone tissue of a Lepidodendron. The whole is surrounded by a sheath of rather wide and short thinner-walled spiral or reticulate tracheids, which may be spoken of as transfusion tracheae, t, and compared with similar elements in the leaves of many recent Conifers. To this tissue Renault applies the epithet “water-bearing” and it is very likely that this may have been its function. The shaded portions of the lamina, in fig. 142, A, represent the distribution of thicker-walled hypodermal tissue. The section of a leaf 3 mm. wide shown in fig. 142, C, shows an almost identical structure; the transfusion tracheae are richly developed especially on the sides and lower212 surface of the vascular strand. This leaf occurs in association with a petrified stem of Sigillaria scutellata[503].

Fig. 200.

• A.  Sigillaria Brardi [= S. elegans Brongn. (39)]. Transverse section of stem.
• B.  Sigillaria Brardi [= S. spinulosa, Renault and Grand’Eury (75)]: c³, outer cortex; x, x², xylem. (After Renault and Grand’Eury.)
• C.  S. Brardi, primary xylem element. (A and C after Brongniart.)
• D.  Leaf of Sigillaria Brardi: g, g, stomatal grooves; ep, piece of epidermis of stem. (After Renault.)
• E.  Sigillaria Brardi. Tangential section of leaf-bases: p, parichnos. (After Renault.)
• F, G, H.  Sigillaria leaf-scars and cushions. (After Weiss.)

Renault[504] has shown that the leaf-traces of Sigillaria spinulosa (= S. Brardi) are accompanied in the outer cortical region of the stem by a fairly large amount of secondary xylem; in sections of the free lamina which he figures the secondary elements are much less obvious and represented by a few tracheae only. Similarly, in the leaf-base of S. Brardi 213(fig. 200, E) the xylem consists of both primary and secondary elements (x, x²), but in the lamina the latter is poorly if at all represented. In the lamina of the leaves of S. Brardi the primary xylem forms a narrow slightly curved band with two lateral groups of narrower, presumably protoxylem elements; this is surrounded by delicate parenchyma styled by Renault, on very slender evidence, phloem (“liber”). Some dark cells below the xylem are described as sclerous tissue, and surrounding the bundle is a sheath of transfusion tracheae (dotted area in fig. 200, E). It is possible that the elements spoken of with hesitation by Renault as secondary xylem are transfusion tracheae.

There has probably been some confusion in the minds of authors between sclerous tissue and dark secretory tissue in Sigillarian leaves; the crescentic band, a, shown in fig. 142, B, which corresponds in position with the sclerous tissue of Renault in S. Brardi leaves, appears to be of the nature of secretory tissue.

The diagram shown in fig. 142, B, illustrates a type of leaf very like those already described, except that there are two xylem strands, x. The difference between the double strand and the single bundle seen in figs. 142, A, C and 200 E, is comparatively small, but it is a real distinction. This type of leaf (fig. 142, B) was originally described by Renault[505] under the generic title Sigillariopsis. The genus was founded on a French petrified specimen consisting of part of a ribbed stem possessing a stele of the Sigillarian type and characterised by separate primary xylem strands, like those of S. Brardi described by Brongniart in 1839. Renault considered the presence of two xylem strands in the leaf a sufficient reason for the institution of a new genus and named the specimen Sigillariopsis Decaisnei. Prof. Bertrand of Lille kindly photographed for me Renault’s type-specimen and sent several prints with explanatory notes. The transverse section of the leaves shows very clearly the two xylem strands; each strand consists of a triangular group of primary tracheae with the protoxylem apex pointing towards the lower surface of the lamina. Below each primary strand 214of centripetal xylem is an arc composed of a few small tracheae which Renault and Bertrand describe as secondary xylem; it is, however, not clear from the photomicrographs that these are of secondary origin, their position and appearance reminding one of the primary centrifugal xylem of a cycadean foliar bundle. Below this centrifugal xylem is another arc of imperfectly preserved elements described by Renault as a protective sheath and by Bertrand as glandular tissue; the latter term is probably the more correct as the tissue may well correspond to the secretory-zone tissue of Lepidodendron stems. Fairly large groups of transfusion tracheids occur on the flanks of the xylem. Prof. Bertrand points out that one of his sections, cut nearer the apex of a leaf than that figured by Renault with a single xylem strand, contains a double strand and thus shows the latter’s description to be an incorrect interpretation of the imperfectly preserved tissues.

The Sigillariopsis type of leaf was recognised by Scott[506] in English material on which he founded the species Sigillariopsis sulcata. In a section which he has recently figured[507] a lacuna below the two xylem strands is described as “representing secretory tissue”; a band of transfusion tracheae almost encircles the pair of bundles.

In a note published in 1907, Kidston[508] demonstrated the association of Sigillariopsis leaves with an undoubted Sigillarian stem of the Rhytidolepis type and expressed his conviction that Renault’s genus is identical with Sigillaria. The correctness of Kidston’s conclusion has been proved by Arber and Thomas[509] who found that the leaf-traces of Sigillaria scutellata bifurcate during their course through the outer region of the cortex and enter the leaf as two distinct strands of primary xylem. In the section from Dr Kidston’s collection shown in fig. 142, B, the lamina, 4 mm. wide, consists mainly of thin-walled assimilating tissue composed of radially elongated cells abutting at the periphery on hypodermal mechanical tissue, except at the edges of the stomatal grooves which are bounded by the small-celled epidermis. A broad sheath of thicker-walled 215elements, s, surrounds numerous scattered transfusion tracheae, t, and below the two xylem strands, x, which are embedded in delicate parenchyma there is a crescentic band of dark tissue, a, resembling the smaller strand, a, in fig. 142, A′, and the secretory zone tissue of a Lepidodendron stem.

iii. Fertile shoots of Sigillaria.

Reference has already been made to the manner of occurrence of strobili on Sigillarian stems; it remains to describe the structure of these reproductive shoots. Sigillariostrobus, the name given to Sigillarian strobili, may be defined in general terms as follows:

Cylindrical cones, rarely dichotomously branched[510] as in species of Lycopodium and Selaginella, which may reach a length of 30 cm. (e.g. Sigillariostrobus nobilis Zeill.[511]) and a diameter 2–5 cm.; peduncle long and slender, sometimes bearing acicular bracts or, after leaf-fall, characterised by leaf-cushions and leaf-scars like those on vegetative shoots (fig. 201, E). The stalked cones are borne in irregular verticils and in some species in vertical series, the fertile zones being separated by comparatively long sterile portions of the stem (fig. 199). The cones were deciduous and, in certain cases if not in all, the individual sporophylls became detached from the cone-axis on maturity. The slender axis bore spiral or verticillate imbricate sporophylls attached at right angles or more or less obliquely. The basal rhomboidal portion bore spores on its upper surface (fig. 201, F), presumably enclosed in a somewhat radially elongated sporangium (fig. B) and was prolonged distally into a narrow lanceolate free portion, in some species with a ciliate border (fig. D). The sporangia probably produced megaspores and microspores, but such spores as have been recognised appear to belong to the former category. The designation Triletes is applied to isolated spores of Sigillaria or to those of Lepidodendron.

Sigillariostrobus Tieghemi Zeiller[512] (figs. 201, E, F). In this 216species, from the Coal-field of Valenciennes, the pedicel bore acicular leaves or bracts attached to the upper portion of leaf-cushions arranged in vertical series (fig. E). The cones reached a length of 16 cm. and a breadth of 2·5–5 cm.; the sporophylls are borne in alternating verticils with 8–10 in each whorl. Several megaspores (2 mm. in diameter) appear to have been produced in tetrads in each sporangium.

Fig. 201. Sigillariostrobus.

• A, C.  Sigillariostrobus rhombibracteatus Kidst. (After Kidston.)
• A.  Portion of strobilus.
• C.  Megaspore.
• B, D.  Sigillariostrobus ciliatus Kidst. (After Kidston.)
• E, F.  Sigillariostrobus Tieghemi Zeill. (After Zeiller.)

Sigillariostrobus rhombibracteatus Kidston[513]. Fig. 201, A, C.

Kidston described this species from the Middle Coal-Measures of England: it is similar in habit and in the form of the sporophylls to S. Tieghemi, but rather smaller, and the more definitely rhomboidal sporophylls have a ciliate margin. The 217cone was probably heterosporous, but megaspores alone have so far been discovered. The sporophylls bear a close resemblance to those of Lycopodium cernuum (fig. 126, C). In some of the illustrations of this type given by Kidston the naked cone-axis with its numerous sporophyll-scars is clearly shown, reminding one of the naked axes of the cones of the Silver Fir (Abies pectinata) or Cedar after the fall of the scales.

Our knowledge of Sigillarian cones is too incomplete to admit of a detailed comparison with the strobili of Lepidodendron or with those of recent Pteridophytes. There can, however, be little doubt that Goldenberg[514] was correct in his selection of Isoetes as the most nearly allied recent plant so far as the fertile leaves are concerned. It would seem that the sporangia were comparatively delicate structures which have left no clearly defined remains of their walls in the carbonised specimens; Kidston, indeed, speaks of the hollow bases of the sporophylls as holding the spores, but this is hardly likely to have been the case. Our knowledge of the anatomy of Sigillariostrobus is practically nil, but in one specimen of a Sigillaria elegans stem Kidston[515] describes the structure of the tissues as seen in a transverse section of a scar of a fertile shoot; from this we learn that the stele was composed exclusively of primary tracheids forming a solid strand without a pith. It is probable that the cones of Sigillaria were heterosporous, but in no instance have undoubted microspores been discovered; the megaspores in each megasporangium were fairly numerous as in Isoetes (fig. 133, E). In one species, Sigillariostrobus major (Germar), from Permian rocks of France and Germany, Zeiller[516] states that the whole of a single cone bore megaspores (0·8–1 mm. in diameter) only; this is, however, not opposed to the idea of heterospory, as we find instances in Selaginella of strobili bearing one kind of spore only (cf. p. 56).

In a few instances, it has been possible to correlate cones with certain species of Sigillaria, but in most cases the strobili occur as isolated fossils.

218

iv. The structure of Sigillarian stems.

The first account of the anatomy of Sigillaria we owe to Brongniart[517] who published a description of the internal structure of an agatised stem, about 4 cm. in diameter, from Autun, which he referred to Sigillaria elegans. It has, however, been shown by Zeiller[518] and by Renault that this petrified fragment belongs to Brongniart’s species S. Menardi, which is probably a young form of S. Brardi. Brongniart’s specimen, now preserved in the Paris Natural History Museum, is a very beautiful example of a silicified plant: on part of the surface are preserved the hexagonal contiguous leaf-scars, like those shown in fig. 193, A, and on the polished transverse section is seen a relatively large stele consisting of a ring of secondary xylem surrounding a series of crescentic groups of primary xylem (fig. 200, A) enclosing a wide pith occupied by concentric layers of silica. A portion of the outer cortex is preserved, and this is separated from the stele by a broad space filled with siliceous rock. The main features of this type may be described in a few words. The primary xylem differs from that of such Lepidodendron stems as have been described in being made up of groups of scalariform and occasionally reticulate (fig. 200, C) tracheae, having a plano-convex or more or less crescentic form as seen in transverse section. These primary strands, in contact with one another laterally, have their narrowest elements on the outer edge. The leaf-traces are given off from the middle of the abaxial face of each xylem strand (fig. 202, C, lt); these pass obliquely outwards through medullary rays and then, as in Lepidodendron, turn sharply upwards before bending outwards again on their way to the leaves. Each leaf-trace consists of a group of primary tracheae to which a few secondary tracheae are added during the passage through the secondary wood. The secondary xylem forms a continuous cylinder of tracheae with scalariform bands on both radial and tangential walls; the medullary rays are numerous and consist of long and narrow 219series, usually one cell broad, of parenchymatous cells with occasional short rays one or more cells in depth.

The slightly greater breadth of the rays between each primary xylem strand tends to divide the secondary wood into bundles corresponding in breadth to the primary groups. The outer cortex closely resembles that of Lepidodendron; it consists internally of radial series of secondary, elongated and rather stout, elements abutting on the parenchymatous tissue of the leaf-cushions.

The next contribution to our knowledge of the anatomy of Sigillaria was made by Renault and Grand’Eury[519] who described the structure of Sigillaria spinulosa Germar[520], a species now recognised as the Leiodermarian condition of S. Brardi, and probably, therefore, not specially distinct from the specimen described by Brongniart in 1839 as S. elegans. In Brongniart’s fossil the leaf-cushions are in contact (Clathrarian form of S. Brardi: fig. 203, upper part) whereas in the specimen now under consideration the leaf-scars are further apart (Leiodermarian form of S. Brardi, fig. 203, lower part, and fig. 196, C). It may be, as Scott suggests, that these two specimens are not specifically identical but closely allied, an opinion based on certain anatomical differences[521]; we may, however, include both under the comprehensive name S. Brardi.

The primary xylem (fig. 200, B, x), is in some regions separated into distinct strands, in others it forms a continuous band equal in length to several of the separate groups. This type of stele, in which the primary xylem consists in part of separate strands and in part of a continuous cylinder, forms a transition between that represented in fig. 200, A, and the steles of Sigillaria elegans (fig. 202, A) and most species of Lepidodendron. The tendency of the primary xylem strands to become united laterally, forming broader bands, was first described by Solms-Laubach[522] in a French specimen of Sigillaria spinulosa in the Williamson collection. The leaf-traces arise from the middle of the concave outer face of the primary xylem groups. The inner cortex is composed of small parenchymatous cells as in Lepidodendron,220 and it is noteworthy that traces of partially disorganised tissue, described as large canals, in the region external to the secondary wood, bear a resemblance[523] to the secretory tissue of Lepidodendron.

Other interesting features are presented by the structure of the outer cortex and the parichnos. The outer cortex in the leaf-scar region is composed of parenchyma, but for the most part it consists of radially elongated groups of thin-walled parenchyma enclosed in a framework of thicker-walled and elongated elements (fig. 200, B, c³). This type of cortex, to which Brongniart applied the name Dictyoxylon, would produce a cast in the case of a partially decorticated stem characterised by a surface formed of irregularly oval and raised areas bounded by narrow grooves; the greater prominence of the former being due to the more rapid decay of the softer tissue, which would 221produce depressions on the exposed face of the dead stem. Casts of this type are not uncommon in Carboniferous rocks, and while some may belong to the Pteridosperm Lyginodendron, others may be those of Sigillarian stems.

Fig. 202.

• A.  Sigillaria elegans Brongn. (Section in Dr Kidston’s Collection cut from the specimen shown in fig. 193, D.)
• B, C.  Sigillaria elongata Brongn. lt, leaf-trace. (From specimens in the collection of Prof. Bertrand.)

The large parichnos-strands, produced as in Lepidodendron, by the forking of a single strand arising in the middle cortical region, consist in part of tissue containing secretory canals, a structure like that recently described by Miss Coward[524] in the large parichnos strands of Syringodendron stems.

An example of a decorticated specimen is described by Renault[525] as Sigillaria xylina. This stem is presumably referred to Sigillaria because the primary xylem consists of separate strands. It is characterised by the unusually large development of secondary wood and by the relatively small size of the pith. The xylem cylinder has a diameter of 4–5 cm. and the pith is only 4–5 mm. in breadth.

Another example of a petrified Sigillaria stem has been described by Kidston[526] as S. elegans Brongn.[527] (fig. 193, D), a species characterised by vertical rows of sub-hexagonal and contiguous leaf-scars and by the presence of verticils of cone-scars. Fig. 193, D, represents Kidston’s specimen in surface-view; one row of leaf-scars is shown, but most of the superficial tissues have been destroyed. The crushed stele, 13 mm. in its longest diameter, has a continuous cylinder of primary xylem, (fig. 202, A, x) characterised by a regularly crenulate outer margin with the smallest elements at the edge; the prominent ridges separating the sinuses are rounded. The leaf-traces arise from the bottom of each sinus; the leaf-bundles are mesarch, and consist exclusively of primary elements. The secondary xylem, x², like that of the primary xylem, has a crenulate outer edge. The most interesting feature of the outer cortex is afforded by a tangential section which, in addition to the leaf-scars, cuts through a cone-scar showing a solid primary stele surrounded by the cortex of the cone-peduncle.

Another type of Sigillaria, probably S. elongata Brongn. (fig. 202, B, C), which is very similar to S. scutellata has been 222briefly described by Prof. Bertrand[528], to whom my thanks are due for the two photographs reproduced in fig. 202, B., C. His specimen, from the Pas de Calais Coal-field, shows a ribbed Rhytidolepis form of surface (fig. 202, B). The stele (fig. 202, C) agrees closely with that of S. elegans as described by Kidston, but the ridges on the fluted surface of the primary xylem are more pointed. “In the immediate neighbourhood of the origin of a leaf-trace, the spiral elements form a median band in the middle of a sinus” and from this the leaf-traces are given off. No secondary xylem was found in the leaf-traces at any part of their course.

Bertrand compares the stele of S. elongata with that of the type of Lepidodendron represented by the Burntisland species named by Williamson L. brevifolium (fig. 186) and now usually referred to L. Veltheimianum; the chief distinguishing features are the greater prominence in the French species of the surface-ridges or teeth of the primary xylem, a feature which occurs in L. Wünschianum, and the detachment of the leaf-traces from the bottom of each sinus (fig. 202, C, lt) instead of from the sides of the sinus. It is, however, not clear how far this latter distinction is a real one; in Lepidodendron Wünschianum the leaf-traces appear to arise, as in Sigillaria, from the middle of each sinus.

Other types of ribbed Sigillaria stems have been briefly described by Scott[529], Kidston[530], and more recently, by Arber and Thomas[531].

The specimen described by Scott agrees in the main with S. elegans of Kidston and with S. elongata of Bertrand.

Kidston’s sections of S. scutellata show a continuous primary xylem cylinder with a slightly and irregularly crenulate outer margin. It would seem that one important diagnostic character in Sigillarian stems is afforded by the degree and form of the crenulations on the outer surface of the primary xylem. S. scutellata has been described also by Arber and Thomas; these authors were the first to demonstrate the presence of a ligule and ligular pit on the leaf-base in a petrified stem, and they 223also contribute the important fact that the leaf-traces in passing through the phelloderm bifurcate and enter the leaf as two distinct vascular strands. This double bundle has been referred to in the description of Sigillaria leaves. (page 214.)

Although our knowledge of the anatomy of Sigillaria has been considerably extended since Williamson[532] drew attention to our comparative ignorance of the subject, there are several points on which information is either lacking or very meagre. As regards the stele, it is in all types so far investigated, of the medullated type and constructed on the same plan as that of Lepidodendron Wünschianum, L. Veltheimianum, and other species. Secondary xylem was developed at an early stage of growth, and its relation to the primary xylem, from which as Kidston points out in his description of S. elegans, it may be separated by a few parenchymatous elements, is like that in Lepidodendron. The tendency of the outer face of the secondary xylem to present a crenulate appearance in transverse sections may, as Scott thinks[533], be a feature of some diagnostic importance, but this is not a constant character in the genus. In origin and in their mesarch structure, the leaf-traces closely resemble those of Lepidodendron. The earlier account of the structure of the leaf-traces of Sigillaria, which were described as possessing both centrifugal and centripetal wood, led Mettenius[534] to draw attention to an important anatomical resemblance between this genus and modern Cycads. This comparison was, however, based on a misconception; the Cycadean leaf-trace, consisting solely of primary wood, is not strictly comparable with those of some species of Sigillaria, in which one part of the xylem is primary and another secondary. The occasional presence of secondary xylem in Sigillarian leaf-traces is matched in some Lepidodendra[535], and cannot be accepted as a distinguishing feature.

The origin of the leaf-traces from the middle of the sinuses on the edge of the primary xylem is regarded as a difference; in Lepidodendron the leaf-traces are said to arise in some species from the sides of the crenulations; but, as already pointed out, 224this is a distinction of doubtful value. The division of the primary xylem into separate strands in some stems of Sigillaria of the Clathrarian and Leiodermarian forms is a characteristic peculiarity; but S. spinulosa forms a connecting link between this type and the continuous arrangement of the xylem in S. elongata and S. elegans. Kidston[536] has shown that the discontinuous primary xylem occurs in Lower Permian species, a fact consistent with the view that the greater abundance of the centripetally developed wood, characteristic of the older species, represents a more primitive feature. This is not merely a conclusion drawn from a consideration of geological age, but it is in harmony with the view expressed by Scott[537] that as plants achieved greater success in producing secondary centrifugal wood, the retention of any considerable quantity of primary xylem became superfluous. As yet we know very little of the structure of the perixylic tissues of Sigillaria, but there is no sufficient reason for supposing that these differ in essentials from those in Lepidodendron. The middle and outer cortical tissues are practically identical in the two genera. The parichnos is of the same type, except that in Sigillaria it reached greater dimensions in the outer part of its course.

v. Sigillaria Brardi[538] Brongniart.
Figs. 196, A–C; 200; 203.

225

Fig. 203. Sigillaria Brardi Brongn. (¾ nat. size). From a photograph of a specimen in Dr Kidston’s collection, from the Upper Transition Series of Staffordshire. Published by Kidston (02) Pl. LIX. fig. 1.

The aerial shoots of this species are occasionally branched dichotomously[539], the apical portions bearing short crowded leaves[540]; the surface of the bark is either completely covered with contiguous leaf-scars without definite leaf-cushions or with projecting cushions forming a narrow sloping surface surrounding each leaf-scar. Other parts of the plant may possess cushions similar in their kite-shaped form to those of Lepidodendron, but without a median vertical groove, or the leaf-scars may be spirally disposed at varying distances apart on a comparatively smooth and longitudinally wrinkled bark. The species exhibits striking instances of a transition between the Favularian, Clathrarian, and Leiodermarian forms of stems. The leaf-scars, which are hexagonal in 226outline,—the lateral angles pointed and transversely elongated, the upper and lower angles rounded,—bear three scars, the central leaf-trace and two straight or curved lateral parichnos scars; a ligular pit occurs immediately above the centre of the upper edge of the leaf-scar and occasionally circular elevations with a central pit occur singly or in pairs below a leaf-scar (fig. 196, A). The linear leaves, which may persist on shoots having a fairly large diameter[541], have a single median vein and two stomatal grooves on the lower surface[542] (fig. 200, D).

Partially decorticated and younger shoots are characterised by the occurrence of pairs of elliptical parichnos areas and a smaller median leaf-trace scar. The surface of older stems, which may show signs of longitudinal splitting (Syringodendron state), bears pairs of parichnos scars reaching a length of 2–2·5 cm. and a breadth of 10–13 mm. The regularity of the leaf-scar arrangement is interrupted at intervals by the occurrence of more or less regular verticils of scars marking the position of deciduous shoots. Grand’Eury[543] has figured cones which he believes to be those of this species, and Zeiller refers the large strobili, Sigillariostrobus major, to Sigillaria Brardi[544].

The subterranean axes were characterised by spirally disposed rootlet-scars like those of Stigmaria ficoides (figs. 204, 205) and by a cortical surface with the features of Stigmaria rimosa Gold.[545]

The anatomy of the stele and leaves has already been described (p. 219). The stele of the Stigmarian portion of the plant consists of a band of centripetal primary xylem and a cylinder of centrifugally formed secondary wood with medullary rays containing vascular bundles passing out to the rootlets[546].

Sigillaria Brardi occurs not uncommonly in Permian rocks; it is recorded from France[547], Germany[548], Pennsylvania[549], and elsewhere. It is found in the Upper, Middle, and Lower Coal-Measures of England[550] and in Permo-Carboniferous strata in Africa[551] and Brazil[552].

Stigmaria.

Stigmaria ficoides is the name given to cylindrical casts met with in Palaeozoic rocks, from the Devonian[553] to the Permian[554], characterised by a smooth or irregularly wrinkled surface bearing spirally disposed circular scars bounded by a raised rim and containing a small central pit. It is not uncommon to find evidence of a partial collapse of the substance of the plant as 228seen in fig. 204; this is doubtless the expression of a shrinkage of the middle cortical region, which was composed of a delicate and lacunar system of cells. There can be no reasonable doubt that Stigmaria grew in water or in swampy ground. Specimens are occasionally met with in which the cast terminates in a bluntly rounded apex; such are, perhaps, young branches which have not grown far from the base of the aerial stem from which they arose (cf. fig. 207, B, C). Other examples occur, such as Goeppert[555] figured and Gresley[556] has more recently described, which are twisted and distorted as though obstacles had been encountered in the ground in which they grew.

Fig. 204. Stigmaria ficoides Brongn. M.S. (See Vol. i. p. 73.)

Fig. 205. Stigmaria ficoides. From a specimen in the York Museum, from Bishop Auckland. a, base of rootlet showing vascular bundle scar. M.S.

The circular scars mark the bases of long single and occasionally forked appendages (rootlets) which spread on all sides into the surrounding medium (figs. 205, 208). The occurrence of rootlets radiating through the shale or sandstone affords proof that the Stigmarias are often preserved in their position of growth. This was recognised by Steinhauer[557] and Logan[558], and has been more recently emphasised by Potonié[559] as 229an argument in favour of the view that the beds containing such specimens are old surface-soils.

Stigmaria usually shows regular dichotomous branching, the arms spreading horizontally or slightly downwards and always arising from four main branches in the form of a cross (fig. 207). The most remarkable specimens found in England are described by Williamson[560] in his monograph of Stigmaria. One of two large casts found near Bradford in Yorkshire, and now in the Manchester Museum, shows four large primary arms radiating from the base of an erect stump 4 feet in diameter. Each arm divides a short distance from its base into two, and the smaller branches extend almost horizontally for several feet[561].

An illustration published by Martin in 1809[562] shows a characteristic feature of Stigmarian casts, namely the presence of a smaller axis, usually occupying an eccentric position inside the larger. This represents the cast of the fairly broad parenchymatous pith which, on decay, left a space subsequently filled by sand or mud: at a later stage the surrounding wood and cortex were removed and the cavity so formed was similarly filled. A thin layer of coal formed by the carbonisation of some of the tissues frequently surrounds the medullary cast, and Steinhauer, whose account of the genus is much fuller and more scientific than those of other earlier and many later writers, recognised the true nature of this internal cast. Artis[563] regarded it as the remains of a young plant, which he described as “perforating its parent,” at length bursting it and assuming its place, a gratuitously drastic interpretation.

In 1838[564] Lindley and Hutton figured a partially petrified specimen of Stigmaria obtained by Prestwich from Carboniferous rock of Shropshire. This example showed a fairly broad cylinder of secondary wood penetrated by medullary rays. The medullated stele consisted of a pith surrounded by a small amount of primary xylem and by a cylinder of 230secondary scalariform tracheae. The preservation of the tissues abutting on the edge of the wood is usually very imperfect, and the middle cortex of lacunar parenchyma has practically in every case eluded the action of mineralising agents; the outer cortex, on the other hand, consists of more resistant elements and is frequently well preserved. As in Lepidodendron and Sigillaria stems, meristematic activity produced a broad band of secondary cortex; and beyond this were attached to cushion-like pads the numerous appendages, each supplied with a single vascular bundle which arose from the primary xylem and passed outwards through a medullary ray. There is abundant evidence that the appendages were hollow, a fact in striking accord with the aquatic and semi-aquatic habitat (cf. Isoetes root, fig. 133, G).

Fig. 206. Cyperus papyrus. Piece of rhizome showing rootlet-scars. Nat. size. M.S.

The piece of dried rhizome of Cyperus papyrus shown in fig. 206 is an almost exact counterpart of Stigmaria ficoides; the wrinkled and shrivelled surface and the circular root-scars containing the remains of a vascular bundle are striking features in common and, it may be added, the two plants, though very different in structure and in systematic position, illustrate anatomical adaptations to a similar environment.

231

Stigmaria ficoides Brongniart[565]. Figs. 204, 205, 207, 208.

The first figure of Stigmaria is said to be by Petver in 1704; Volkmann published illustrations of this common fossil in 1720 and Parkinson in 1804[566]. Binney, whose researches may be said to have inaugurated a new era in the investigation of fossil plants, wrote in 1844: “Probably no fossil plant has excited more discussion among botanists than the Stigmaria. It is the most common of the whole number of plants found in the Coal-Measures, but there has hitherto been the greatest uncertainty as to its real nature[567].” This uncertainty still exists, at least in the minds of some who know enough of the available data to realise that our knowledge is imperfect.

To pass to the questions of the affinity and nature of Stigmaria: Brongniart[568] at first compared his genus with recent Aroideae, but he afterwards[569] spoke of it as probably the root of Sigillaria. Other writers regarded Stigmaria as a dicotyledonous plant comparable with Cacti and succulent Euphorbias. For many years opinion was divided as to whether Stigmaria represents an independent and complete plant or the underground system of Sigillaria.

Artis[570], Lindley and Hutton[571], as well as Goldenberg[572], believed it to be a prostrate plant unconnected with any erect aerial stem. Goldenberg figured one of the slender rootlets terminating in an oval body described as a reproductive organ. This seed-like impression is either some extraneous body or an abnormal 232development at the end of a rootlet. In 1842 Logan drew attention to the almost complete monopolisation by Stigmaria of the underclays, the rock which as a general rule occurs below a seam of coal. He wrote: “The grand distinguishing feature of the underclays is the peculiar character of the vegetable organic remains; they are always of one kind (Stigmaria ficoides) and are so diffused throughout every part of the bed, that by their uniform effect alone the clay is readily recognised by the eye of the miner[573].” This fact, which has played a very conspicuous part in the perennial discussions on the origin of coal, led to the almost general recognition of the underclays as surface-soils of the Coal period forests.

The next step was the discovery of Stigmaria in the Coal-Measures of Lancashire and in the Carboniferous rocks of Cape Breton, Nova Scotia, forming the basal branches of erect stems identified by Binney[574], Bowman[575] and Richard Brown[576] as undoubted Sigillariae. In one case Brown found what he considered to be convincing evidence of the continuity between Stigmaria and Lepidodendron.

In 1842 Hawkshaw[577] described certain fossil trees, the largest of which had a circumference at the base of 15 ft., discovered, in the course of excavations for a railway in Lancashire, in soft shale at right angles to the bedding. The surface features were not sufficiently clear to enable him to decide with certainty between Sigillaria and Lepidodendron, but while inclining to the former, it is interesting to note that the occurrence of numerous Lepidostrobi near the root led him to recognise the possibility of a connexion between the Stigmarian roots and Lepidodendron stems. In 1846 Binney gave an account of similar trees found at Dukinfield near Manchester: he spoke of one stem as unquestionably a Sigillaria with vertical ribs, furrows, and scars, about 15 inches high and 4 ft. 10 inches in circumference. He expressed his conviction that “Sigillaria was a plant of an aquatic nature[578].” Similar descriptions of rooted stems in the Coal-Measures of Nova Scotia were published by Brown in 2331845, 1846 and 1849; in the last paper he figured a specimen, which has become famous, showing a Syringodendron stem terminating in branching Stigmarian (or possibly Stigmariopsis) roots bearing on the lower surface a series of what he called conical tap roots[579]. A similar specimen discovered in Central France nearly fifty years later demonstrated the accuracy of Brown’s description.

Despite these discoveries the root-like nature of Stigmaria was not universally accepted. It was, however, generally agreed that Stigmaria formed the roots of Sigillaria; it was, moreover, held by some that Lepidodendron stems also possessed this type of root, an opinion based on Brown’s record and on the occurrence of Stigmaria in beds containing Lepidodendron but no Sigillaria stems, as in the volcanic beds of Arran and elsewhere, and on observations of Geinitz and others[580]. There is now general agreement that Lepidodendron and Sigillaria had the same type of “root,” though the connexion of Stigmaria with the former was not so readily admitted, and indeed the evidence in support of it is still very meagre. Goeppert and other authors were unable to believe that the numerous species of Sigillaria possessed roots of so uniform a type, but Goeppert, by his recognition of several varieties of Stigmaria, supplied a partial answer to this objection.

Messrs Mellor and Leslie[581] have described and figured some large casts of roots exposed in Permo-Carboniferous rocks in the bed of the Vaal river at Vereeniging (Transvaal) which exhibit certain features suggesting comparison with Stigmaria. Some of these reach a length of 40–50 feet and, when complete, were probably not less than 100 feet long: in some of them the centre of the cast from which forked arms spread almost horizontally shows a depression in the form of a cross indicating a regular dichotomous branching like that of Stigmaria. The authors incline to the belief that the roots belong to Noeggerathiopsis and not to a lycopodiaceous plant, though Lepidodendroid stems are abundant in the sandstone a few feet 234higher in the series. Despite the absence of any Stigmarian scars on the surface of the fossil it is probable that these fine specimens are the rhizomes of some lycopodiaceous plant, possibly Bothrodendron, which is not uncommon in the Vereeniging beds.

Admitting that Stigmaria is part of Sigillaria, the next question is, is Stigmaria a root in the ordinary sense, the underground system formed on germination of the spore and of equal age with the shoot, or did it bear a different relation to the Sigillarian stems? To this question different answers would still be given. Goeppert[582] discussed evidence in favour of the view that aerial Sigillarian shoots were produced as vegetative buds on pre-existing Stigmarian axes, like young moss plants on a protonema. At a later date Renault[583] developed a similar view as regards Sigillaria; but we may pass on to consider the more recent and complete observations of Grand’Eury[584] and Solms-Laubach[585].

The recognition of two distinct types of Stigmariae in the Coal-Measures of Central France led Grand’Eury[586] to institute a new genus, Stigmariopsis. This type, which is characterised by a difference in habit as well as by other distinguishing features, is represented by such specimens as those figured by Goldenberg as Stigmaria abbreviata, bearing lenticular scars spirally disposed on a cortical surface characterised by irregular longitudinal wrinklings. Stigmariopsis has frequently been found in direct continuity with Sigillarian stems of the Leiodermarian-Clathrarian type, spreading obliquely downwards in the form of rapidly narrowing arms clothed with slender and usually simple appendages; and from the under surface of these arms short conical outgrowths are given off. It is probable, as Solms-Laubach believes, that Stigmariopsis was represented also by long horizontally creeping rhizomes[587] of uniform breadth from which ribless Sigillarian aerial shoots arose as bud-like outgrowths. Grand’Eury, the author of the genus, confined the term to the shorter and more rapidly tapered organs spreading 235from the base of erect stems; the horizontal rhizomes of all Sigillarian stems he refers to Stigmaria. The pith-casts of Sigillariopsis may be recognised by their long vertical ridges and grooves, a feature readily understood by reference to the stem structure. The Stigmariopsis rhizomes though rare in England have been recognised by Dr Kidston[588] in the Middle Coal-Measures of Yorkshire; he has figured a pith-cast very like that illustrated in Solms-Laubach’s Memoir as Stigmariopsis anglica.

The surface-features of a Stigmariopsis pith-cast are clearly shown on a specimen from St Étienne in the Williamson collection[589].

The most complete account of Grand’Eury’s views in regard to the anchoring and absorbing organs of Sigillaria is given in his monograph on the Coal-field of Gard[590], St Étienne, and these are clearly stated also by Solms-Laubach[591] who confirms the conclusions of the French author as to the manner of development of the aerial shoots. Grand’Eury believes that both Stigmaria and Stigmariopsis are rhizomes and not true roots. The surface-features of Stigmaria have already been described. This type Grand’Eury speaks of as characterised by the uniform diameter and considerable horizontal elongation of the bifurcated axes; he thinks they grew both as floating rhizomes and on the ground: they may frequently be traced for a considerable distance without showing any signs of connexion with aerial shoots, but occasionally they have been seen in organic union with Sigillarian stems. He believes that these rhizomes were produced as the result of germination under water of the spores of Sigillaria or Lepidodendron and developed as long and branched aquatic rhizomes capable of independent existence. Under certain conditions, as he thinks in shallower water, the rhizomes produced bulb-like outgrowths which grew into erect stems having the surface-features of Sigillaria. This method of origin is practically the same as that described by Goeppert in 1865. The vascular medullated cylinder of these erect 236branches was in direct continuity with that of the Stigmarian rhizomes.

Fig. 207. An early stage in the development of Sigillaria.
A. Surface-features enlarged. (After Grand’Eury.)

Fig. 208. Later stage in the development of Sigillaria; Syringodendron with Stigmariopsis. (After Grand’Eury.)

The next stage is that in which the undifferentiated bulb becomes swollen at the base and develops four primary roots (fig. 207 B, C) which grow obliquely downwards and produce numerous rootlets. Meanwhile the parent rhizome gradually decays, finally setting free the aerial stems which are now provided with spreading and forked roots (fig. 208) such as we are familiar with in English specimens as Stigmaria ficoides, but which in the French specimens show the features of Stigmariopsis. At this later stage conical outgrowths are formed from the under surface of the Stigmariopsis arranged in a more or less regular series surrounding the centre of the forked and spreading roots (fig. 209). These conical and positively geotropic organs were long ago described by Richard Brown as237 tap-roots. Grand’Eury’s conclusions are briefly as follows: Sigillaria, and we may add Lepidodendron, had no true roots and in this respect are comparable with Psilotum (fig. 118): the organs which are described by Grand’Eury as roots are correctly so named in a physiological sense, but morphologically they do not strictly conform, either in origin or in the arrangement of their appendages, to true roots. The question as to whether they are entitled to the designation root is one which it is needless and indeed futile to discuss in detail; it would be conceding too much to a formal academic standpoint to refrain from applying to them the term root, as that best describes their share in the life of the Sigillarian stems. The horizontal Stigmarian axes are rhizomes in the ordinary sense of the term and from these were developed Sigillarian shoots, characterised in the lower portions by large parichnos strands. From the base of the young bulbous shoots roots were formed: these roots being, in the French specimens, of the Stigmariopsis type.

Fig. 209. Stigmariopsis and “tap-roots.” (After Grand’Eury.)

These conclusions require some modification when applied to British representatives of the arborescent Lycopodiales. The long spreading and dichotomously branched root-like organs attached to the base of Sigillarian and Lepidodendron stems are true examples of Stigmaria ficoides or other species. Stigmariopsis occurs but rarely. This marked difference between French and English specimens may be explained if we adopt the opinion of Solms-Laubach, who believes that the true Stigmaria represents both the parent rhizome and the later-formed roots of the Rhytidolepis Sigillarian species and of238 Lepidodendron, the Stigmariopsis form having the corresponding relation to the Leiodermarian-Clathrarian species.

The opinion expressed by Williamson[592] in 1892 that Grand’Eury’s hypothesis “appears to be identical with the vague and speculative guesses that were prevalent among us in the early years of the present [nineteenth] century” illustrates the strength of conviction based on English specimens as to the root-nature of Stigmaria.

There is undoubtedly considerable confusion, which can be cleared up only by further research, as to the precise relation between Stigmaria and Stigmariopsis on the one hand and the different types of Sigillariae on the other. The main contention, and this is the most important point, of Renault, Grand’Eury and Solms-Laubach as to the manner of formation of the aerial shoots from rhizomes and the subsequent production of forked roots and their ultimate separation from the parent rhizome is, as I believe, correct. Williamson held that Stigmaria must be regarded as a true root; he found no evidence to support the view that the large rooted stem discovered by Hawshaw, Binney, and others had been originally produced from aquatic rhizomes. It must, however, be remembered that Grand’Eury’s opinion is based on evidence afforded by the exceptionally well displayed Sigillarian forests of St Étienne, on a scale such as English strata have not as yet afforded. Moreover, the absence of any parent-rhizome in association with the rooted stumps described by Williamson and by others is not a serious argument against their rhizome origin.

The specimen represented in fig. 209, which was examined in situ by Solms-Laubach and Grand’Eury, shows a Sigillarian stem in the Syringodendron condition bearing rows of paired parichnos scars; from the base forked and rapidly tapering arms radiate through the surrounding rock and, as shown by other specimens, these bear numerous appendages like those of the English Stigmarias. The surface-features of the arms are those of Stigmariopsis and the centre of each, as seen on the broken face, is occupied by a pith-cast characterised by parallel longitudinal ridges resembling those on the medullary casts of 239Calamites. It is noteworthy that the petrified rhizome originally described by Renault as Stigmaria flexuosa, and afterwards identified by him as the subterranean system of Sigillaria Brardi, possesses a vascular cylinder composed of primary xylem strands of crescentic transverse section lining the pith; a cast of the pith, after the removal by decay of its delicate parenchymatous tissue, would exhibit the surface-features of Stigmariopsis. Stigmaria flexuosa no doubt represents a true Stigmariopsis rhizome. On the other hand, as Williamson has shown, the inner surface of the wood of Stigmaria ficoides consists of a reticulum of xylem with meshes of medullary-ray tissue; a cast of such a surface presents a very different appearance from that of Stigmariopsis.

Returning to fig. 209: from the lower surface of the Stigmariopsis arms numerous conical outgrowths, reaching a length of several centimetres, project vertically downwards; these also possess Stigmariopsis pith-casts and are identical with the “tap-roots” of Richard Brown. The stump seen in fig. 209 shows the characteristic hollow base of the erect stem: this is the region which, it is believed, represents the position of the Stigmarian rhizome from which the aerial shoot was developed. Although no remains of the parent rhizome were found, traces of the rootlets which probably belonged to it were found in the neighbourhood. The absence of the actual rhizome is, however, not surprising as it would not persist after its aerial Sigillarian branches had attained independence by the development of their own dichotomously branched absorbing and holdfast organs.

The Stigmarian axes of Palaeozoic Lycopods are compared by Miss Thomas[593] with the prop-roots of certain recent flowering plants which grow in tropical tidal swamps; their roots grow downwards from the stem at an angle of 50°-60° before spreading out horizontally. This author also makes some interesting suggestions in regard to the evidence afforded by anatomical structure as to the habitat of Sigillaria and Lepidodendron.

240

Anatomy.

The more important anatomical features of Stigmaria must be dealt with briefly. Williamson’s monograph, published in 1887[594], is considerably in advance of the work of that of any of the numerous writers who had previously dealt with the subject. The diagrammatic transverse section reproduced in fig. 210, H, illustrates the general arrangement of the tissues. The medullated stele was described by Williamson as consisting entirely of centrifugally developed secondary xylem and distinguished, therefore, from the stele of a Lepidodendron or Sigillaria by the absence of a centripetally produced primary xylem zone. The secondary xylem tracheae are characterised by scalariform pits on both radial and tangential walls and, as shown in a figure given by Solms-Laubach[595], the spaces between the transverse bars are bridged across by fine threads, as in the tracheae of Lepidodendron.

One of the largest specimens of a petrified Stigmaria which I have seen is one lent to me by Mr Lomax from the Coal-Measures of Halifax in which the flattened transverse section measures 18 cm. × 3·5 cm., the cylinder of wood being 1·1 cm. × 7 mm. in diameter.

In French examples of Stigmaria or Stigmariopsis it has been demonstrated by Renault[596] that primary xylem strands occur very like those in the stem of some species of Sigillariae (see p. 219). If a well-preserved section of an English Stigmaria is examined it will be seen that the edge of the secondary wood consists of a few narrower elements which do not exhibit the radial seriation characteristic of secondary elements.

A type of Stigmaria characterised by centripetal primary wood has been described by Weiss[597] and referred by him to Bothrodendron mundum; the main results of his observations are stated in the account of Bothrodendron on a subsequent page. This discovery is of considerable interest not only as rendering our knowledge of Bothrodendron remarkably complete but as confirmatory of Renault’s account of French Stigmarian axes in 241which centripetal primary wood is well developed between the secondary xylem and the centre of the stele. The Stigmarian axis of Bothrodendron was originally figured by Williamson as Lepidodendron mundum[598]. The chief difference between Weiss’s specimen and those described by Renault[599] as the Stigmarian axes of Sigillaria Brardi, is that in the English plant the centripetal wood forms a cylinder of uniform breadth 242instead of a band with a crenulated inner margin as figured by Renault.

Fig. 210. Stigmaria.

• A.  Transverse section of vascular bundle of rootlet and part of outer cortex. t, tracheae. (After F. E. Weiss.)
• B, C.  Vascular bundle of rootlet; in C a series of small tracheae are shown extending from the protoxylem.
• D.  Rootlets from the outer cortex of E.
• E.  Part of a large Stigmaria: St, stele; s, intruded rootlet.
• F.  Vascular bundle and tracheae passing obliquely towards the outer cortex, c³.
• G.  Outer cortex of Stigmaria.
• H.  Diagrammatic section of Stigmaria: p, phelloderm; r, rootlets.

An interesting agreement between the French and English specimens is the occurrence in the cortex of groups of reticulate elements: in Weiss’s section these are short and wide and occur in the middle cortex; in Renault’s plant they are more fusiform and occur in the secondary cortical tissue. These elements appear to have been arranged as an interlacing network in the middle cortex and were in close connexion with the rootlet-bundles, comparable, as Weiss points out, with the transfusion tracheids accompanying Lepidodendron leaf-traces.

It is probable that these short and wide tracheal elements served for water-storage and thus afford another indication of the xerophilous character of the Carboniferous Lycopods, a feature possibly connected with a salt-marsh habitat.

The presence of conspicuous medullary rays gives the secondary xylem of Stigmaria the appearance of being divided into several more or less distinct groups (fig. 210, E, St). In tangential longitudinal section the xylem assumes the form of a broad reticulum with lenticular meshes filled with medullary-ray tissue through which strands of xylem are cut across in a transverse direction as they pass outwards from the inner edge of the wood to supply the rootlets. In addition to these broader or primary medullary rays, there were numerous secondary rays composed of narrow plates of parenchymatous cells one or several elements in depth. As Williamson pointed out, the medullary-ray tissue consists in part of radially elongated tracheal elements with spiral or scalariform thickening bands like those described in the same position in Lepidodendron stems.

Our knowledge of the minute structure of the tissues abutting on the secondary xylem is far from complete.

The xylem is succeeded by a zone of delicate cells which was the seat of meristematic activity. It is noteworthy that in a section figured by Williamson[600] there is the same disparity in size between the outermost elements of the xylem and the adjacent cells of the meristematic zone as in Lepidodendron stems. Beyond this region an imperfectly preserved lacunar 243tissue occurs like that which I have called the secretory zone in Lepidodendron stems; but information as to the structure of this part of Stigmaria is much more incomplete than in the case of the aerial shoots. The middle cortex was of the same lacunar type as in the stems, and the fact that it is never well preserved in large Stigmarian axes suggests that it may have been even more richly supplied than in the aerial stems with an aerating system of spaces. The outer cortex, consisting in young examples of large-celled parenchyma, became at an early stage of growth the seat of cambial activity which resulted in the production of radially placed series of secondary elements (fig. 210, H, p). The outer and older elements of this secondary cortex are more tangentially stretched than the inner cells, a necessary result of the position of the phellogen on the internal edge of the tissue and of the increasing girth of the axis.

In comparatively young Stigmarian axes the outer cortex already possesses a band of secondary radially disposed cells characterised by the greater tangential extension of the more external elements; usually this tissue terminates abruptly on the inner edge and the line of separation no doubt marks the position of the phellogen. Occasionally some delicate secondary elements are preserved internal to the phellogen, and these in young specimens form a narrow cylinder composed in part of radially elongated cells showing signs of recent tangential divisions. In its earlier stage of activity the phellogen seems to form a greater amount of secondary tissue on the outside, but this is clearly not of the nature of cork, the tissue which occupies a corresponding position in recent plants. The primary cortex shows no signs of shrinkage or collapse as would be the case were it cut off from the vascular system by a zone of impermeable cork.

Fig. 210, G, represents a piece of the external tissue of a specimen in which the slightly flattened xylem cylinder measures 1·4 × 1 cm.; the inner cortex has disappeared and fragments only of the middle cortex are preserved. The outer cortex, with an average breadth of 2 mm., consists superficially of primary parenchyma with a somewhat uneven surface and with a rootlet attached here and there; a short distance below the244 surface is a band of conspicuous cells, b, characterised by dark contents suggesting very imperfectly preserved fungal hyphae, but the nature of the substance filling the cells cannot be made out with certainty. It is, however, interesting to find that this dark band constitutes an obvious feature (fig. 210 H, b); its position is comparable with that of the dark-walled cells in the outer cortex of rootlets. A short distance internal to this dark band tangentially elongated cells form the outermost elements of the secondary cortex; these become gradually narrower towards the interior and pass into radial series of smaller cells of uniform size, as seen on the inner edge of fig. 210, G. At the inner boundary of this tissue, just below the region shown at the bottom of the drawing, was situated the phellogen. Such traces of tissue as occur on the inner side of the line where splitting has usually occurred, consist of thinner elements with recently formed tangential walls and probably represent an early stage in the development of phelloderm.

A much older section is shown in part in fig. 210, E. The secondary xylem cylinder, St, is shown in the lower part of the section; beyond this is a band of secondary tissue which reaches in some places a breadth of 6 cm. The greater part of this tissue consists of phelloderm of very uniform structure made up of radial series of cells: this is interrupted in most parts of the section by a gap crowded with intruded rootlets (a portion of this is enlarged in fig. 210, D). Beyond this gap the secondary tissue consists of radial series of cells characterised by the considerable tangential elongation of many of the elements, precisely like the tissue figured by Williamson. In all probability the gap represents a line of weakness due to the phellogen, and if this is the case it is clear that in an old Stigmaria the phelloderm exceeded in amount the tissue formed external to the phellogen. The secondary tissue on the inner side of the phellogen is characterised by numerous irregular concentric lines superficially resembling rings of growth in the wood of a Conifer: these are, however, not the result of any periodic change in external conditions, but are apparently due to crushing of the tissue and are possibly, to some extent, the result of the presence of secretory strands like those in the245 phelloderm of Lepidodendron. The surface of this older rhizome retains patches of primary tissue, and an occasional rootlet, as at r, fig. 210, E, is seen in connexion with the cortex; the cortex has been vertically fissured as the result of secondary growth and presents an appearance like that shown in Lepidodendron Wünschianum and L. Veltheimianum (figs. 181, A, and 186, A).

The form in which a Stigmarian rootlet is usually preserved is shown in fig. 210, D; the single vascular bundle strand with its endarch protoxylem (fig. 210, B, px) is enclosed by a ring of inner cortical parenchyma (fig. 210, F, c¹); the cells in immediate contact with the xylem having usually disappeared. Beyond the middle cortical space a second cylinder of parenchyma represents the outer cortex (F, c³) in which a layer of dark-walled cells (b, fig. 210, F) may be compared with the hypodermal band in the main Stigmarian axis (G, b). These Stigmarian rootlets, usually less than 1 cm. in diameter, are the commonest objects in sections of the calcareous nodules from English coal-seams. A good example of their abundance is shown in fig. 210, D and E; here they have invaded the space formed by the splitting of the secondary cortical tissues along the line of the phellogen and a few are seen here and there in the deeper layers of the phelloderm (s, fig. 210, E). Not infrequently the close contact of these ubiquitous rootlets with the tissues of the plant which they have invaded leads to confusion between invader and invaded. Partially decayed tissues lying, probably, under water were penetrated by Stigmarian rootlets in exactly the same way as the roots of recent plants bore through vegetable substances which happen to be in their path. The rootlet bundles are in the first instance composed of the primary tracheae which line the inner edge of the secondary xylem; these receive additions from the meristematic zone, and thus, when seen in the cortex outside the stelar region, are found to consist in part of primary and in part of a fan-shaped group of secondary tracheae. On the other hand, the monarch bundle as it appears in a free rootlet is usually composed entirely of primary elements (fig. 210, A–C, F). It has been shown by Weiss[601] that in the Stigmarian rhizome of what is probably 246Lepidodendron fuliginosum, the rootlet bundle is accompanied by a parichnos strand, but this has not been detected in the ordinary Stigmaria ficoides. When free from the parent axis a rootlet usually consists of an outer cylinder of cortex enclosing a broad space in which remnants of lacunar tissue are sometimes seen. The relation of the external features of a well-preserved Stigmarian rootlet-scar to the internal structure of a petrified rootlet is very clearly seen on comparing such sections as those represented in fig. 210, D, with the form of the scar on a Stigmarian cast. A specimen figured by Hooker[602] in 1848 affords a good illustration of the structure of a rootlet-base as seen in an unusually complete cast; this correlation of anatomical and surface features is clearly described also by Williamson[603] and by Solms-Laubach[604]. It is probable that even during life the rootlets were hollow for a part at least of their length as are the roots of Isoetes (fig. 133, G).

An interesting discovery was made a few years ago which confirmed a statement by Renault which Williamson was unable to accept, namely that the xylem bundle of a rootlet occasionally gives off a delicate tracheal strand at right angles to the long axis of a rootlet. In some rootlets Weiss[605] found obliquely running delicate strands of xylem, surrounded by a layer of parenchymatous tissue, in the space between the vascular bundle and the outer cortical cylinder. It is clear that a few spiral tracheids are occasionally given off from the protoxylem of a rootlet bundle: these follow an oblique course to the outer cortex, where in some cases they have been traced into connexion with short and spirally marked cells resembling transfusion tracheae (fig. 210, A). This arrangement may serve as a means of facilitating the passage of water absorbed by the superficial cells into the xylem strand. It should be noticed that, like roots of recent water-plants, the rootlets of Stigmaria had no root-hairs. Fig. 210, F, shows a transverse section of part of a rootlet in which the outer cortical cylinder, c³, is connected, as in the roots of Isoetes, with the sheath surrounding247 the vascular bundle. A few obliquely cut tracheae are seen in this section traversing the connecting band of parenchyma t, fig. 210, A.

A point of biological interest in connexion with Stigmaria rootlets is the occasional presence of hypertrophied cells, the large size of which is due to the attacks of a fungus named by Weiss[606] Urophlyctites stigmariae.

In addition to Stigmaria ficoides, which is by far the commonest form, a few other species have been founded on external characters. One of these is represented by Stigmaria stellata, Goepp.[607], characterised by the presence of radially disposed ridges and small tubercles surrounding each rootlet-scar. Kidston refers to Goeppert’s species as a Lower Carboniferous type. We have no evidence as to the meaning of the stellate ridges and tubercles, nor have we any reason to suppose that this form differed essentially in structure from Stigmaria ficoides.

Bothrodendron. Figs. 211–216.

Although in many respects the genus Bothrodendron agrees very closely in habit and in its anatomical features with Lepidodendron, there are reasons for referring it to a distinct family of Palaeozoic Lycopods. As the following description shows, the external features do not differ in any essential points from those of certain types of the genus Sigillaria, particularly such a species as S. rimosa, Gold.[608], which has recently been refigured and described by Nathorst[609] from Goldenberg’s type-specimen in the Stockholm Museum. The small size of the leaf-scars is, however, a characteristic feature of Bothrodendron (fig. 212, F); but a more important point is the fact that in a recently described[610] English example of a cone of Bothrodendron (fig. 216), the sporangia are very like those of recent Lycopods, and differ from the radially elongated sporangia of Lepidostrobus. On the other hand, a French cone described by Zeiller[611] as Lepidostrobus Olryi, which is probably a strobilus of Bothrodendron, has the radially elongated type of sporangium (fig. 212, E). The comparative abundance of Bothrodendron in Lower Carboniferous and Devonian rocks points to the greater antiquity of this member of the Lycopodiales as compared with Lepidodendron.

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The name Bothrodendron was instituted by Lindley and Hutton[612] for impressions of stems from the English Coal-Measures, characterised by two opposite rows of large depressions like those shown in fig. 211 and, in one of the specimens, by “a considerable number of minute dots, arranged in a quincuncial manner.” The minute dots were recognised as leaf-scars and the cup-like cavities were described as probably connected with the occurrence of large cones. On very slender evidence this Palaeozoic plant, which was named Bothrodendron punctatum, was considered by these authors as probably a member of the Coniferales. The large stem from the Coal-Measures in the neighbourhood of Mons, Belgium, shown in fig. 211, affords a good illustration of Bothrodendron in a partially decorticated condition, exhibiting a row of depressions similar to those on the Ulodendron form of Lepidodendron Veltheimianum (fig. 157), but distinguished by the eccentric position of the scar at the bottom of each cup-shaped cavity: in the Belgian specimen, which is partially decorticated and shows the leaf-traces as small dots, the depressions have a diameter of 9 cm. It is believed by some authors that these Ulodendron shoots of Bothrodendron and Lepidodendron owe their characteristic appearance to the pressure of large cones, but, as I have already stated, there are reasons for preferring the view that these crater-like hollows are the scars of deciduous branches. Our knowledge of the strobili borne by Bothrodendron stems is still meagre, but we have no reason to assume the existence of any cones large enough to produce by the pressure of their bases such depressions as those shown in fig. 211. In one species at least the strobili were borne terminally on slender shoots (fig. 213). The Ulodendron condition has so far been recognised in one species only, B. punctatum.

In his catalogue of Palaeozoic plants, Kidston[613] included Bothrodendron punctatum as a synonym of Sigillaria discophora König, a mistake which he afterwards rectified[614]: the generic name Bothrodendron was generally ignored by authors in the belief that the specimens described by Lindley and Hutton 251were not generically distinct from the fossils originally figured by Rhode as Ulodendron. It was Prof. Zeiller who first demonstrated that the English authors were justified in their choice of a new designation for stems with large depressions in association with minute leaf-scars. In 1859 Haughton[615] proposed a new family name Cyclostigmaceae for some Upper Devonian plants from County Kilkenny, Ireland: he described three species of his new genus Cyclostigma, Cyclostigma kiltorkense, C. minutum, and C. Griffithsi; these are now generally recognised as a single species of Bothrodendron, though, as Nathorst suggests, the Irish plant should perhaps be separated as a sub-genus Bothrodendron (Cyclostigma) by reason of certain minor differences which distinguish it from other species of the genus.

Fig. 211. Bothrodendron punctatum. Part of a specimen from near Mons (Hainaut), in the Brussels Museum. (Reduced.)

Another generic name, Rhytidodendron, was instituted by Boulay in 1876 for stems characterised by a finely wrinkled bark and small spirally disposed leaf-scars. A short description of this type, which occurs in the Middle and Lower Coal-Measures, may serve to illustrate the external features of the commonest British example of the genus.

a. Bothrodendron minutifolium (Boulay.) Figs. 212, A, C, D; 213.