Chapter XV: Arborescent Lycopodiales (2)

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 spirally 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 from 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 of 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.”

There has been no lack of ingenuity on the part of authors in 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.

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 specimen 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 the 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.

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 overlooked 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.

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.

A. _Lepidophloios scoticus_ Kidst. From a specimen from the
Calciferous Sandstone, Midlothian, in Dr Kidston’s Collection;
rather less than ⅓ nat. size.
B. _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 decorticated 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.

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_ _regularis_ 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 is 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 _Halonia_; 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 with 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.

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.

1871. _Lepidodendron Harcourtii_, Binney, Palæont. Soc., p. 48, Pl.
VII. fig. 6.
1872. _Halonia regularis_, Binney, Palæont. Soc., p. 89, Pl. XV.
1881. _Lepidodendron Harcourtii_, Williamson, Phil. Trans. Roy.
Soc., Vol. 172, p. 288, Pls. XLIX–LII.
1887. _Lepidodendron fuliginosum_, Williamson, Proc. Roy. Soc.,
Vol. XLII. p. 6.
1891. _Lepidodendron Williamsoni_, Solms-Laubach, Fossil Botany, p.
226.
1893. _Lepidophloios fuliginosus_, Kidston, Trans. Roy. Soc.
Edinburgh, Vol. XXXVIII. p. 548.

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 only 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_ and _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.

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].

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 and 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, _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. 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.

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).

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, _a_ 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_.

A. _Lepidodendron vasculare._ (Botany School, Cambridge.)
B. _Lepidodendron fuliginosum._ (From a specimen from Shore,
Lancashire, in the Cambridge Botany School Collection).
C. _L. fuliginosum._ (“Biseriate Halonia” of Weiss No. 257,
Manchester University Museum.)
D. _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 margin 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 tracheae (fig. 168, D, _x_², and fig. 170, A, _x_²), or it may consist of parenchyma containing scattered groups of tracheae (fig. 169, A, _x_²)[326].

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, _x_², consisting of parenchyma and tracheae, passes into the cambial band (fig. 169, B, _a_).

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. The 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 a Lepidodendron shoot. This applies to other species of the genus as well as to _L. fuliginosum_.

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, _x_²; the primary stele has a diameter of 1 cm.; the secondary xylem, _x_², 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.

The secretory zone of _Lepidodendron fuliginosum_ agrees essentially with that of other species; it usually presents the 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, _c_², 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 specimen 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_.

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 secondary 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 tissue 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.

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.

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 well 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].

_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 afterwards 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, _c_³, 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 invaded 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 _x_², 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 _c_¹, fig. 177, B, is seen the inner cortical tissue.

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 points 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 Mr 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, _c_³, 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.

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 which 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.

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 visited 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 xylem, _x′_, occur several isodiametric tracheae with fine scalariform and reticulate thickening bands like those in the central region 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.

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 of secondary xylem, _x_², 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.