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.
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.
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.
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.
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.
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.
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.
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, x2, and fig. 170, A, x2), or it may consist of parenchyma containing scattered groups of tracheae (fig. 169, A, x2)[326].
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).
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, 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.
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, 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 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.