Chapter XV: Arborescent Lycopodiales (3)

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 of 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 secretory 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, _x_²). 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 as 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).

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 suggest 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.

1820. “Schuppenpflanze,” Rhode, Beit. zur Pflanzenkunde der
Vorwelt, Pl. III. fig. 1.
1825. _Lepidodendron Veltheimianum_, Sternberg, Flora der Vorwelt,
Pl. LII. fig. 5.
1836. _Pachyphloeus tetragonus_, Goeppert, Die fossilen
Farnkräuter, Pl. XLIII. fig. 5.
1852. _Sagenaria Veltheimiana_, Goeppert, Foss. Flora des
Übergangsgebirges, Pls. XVII–XXIV.
1875. _Lepidodendron Veltheimianum_, Stur, Culm Flora, p. 269, Pls.
XVIII–XXII.
1886. _Lepidodendron Veltheimianum_, Kidston, Catalogue of
Palaeozoic plants, British Museum, p. 160.
1901. _Lepidodendron Veltheimianum_, Potonié, Silur und Culm Flora,
p. 116, figs. 72–76.
1904. _Lepidodendron Veltheimianum_, Zalessky, Mém. Com. Géol.
Russie, Pl. IV. figs. 4, 5.
1906. _Lepidodendron Veltheimi_, Potonié, Königl. Preuss. geol.
Landesanstalt, Lief. III.

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 may be that their prominence in the specimen before us is the result of the decay of a thin layer of superficial cortex which hid 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. 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_.

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 megaspores 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 both 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_.

A, B. _Lepidodendron Veltheimianum._ (Botany School, Cambridge.)
C. _Lepidodendron macrophyllum._ (British Museum. No. 377.)
_x_, Primary xylem; _x_², 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 published 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 distance 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_.

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, assuming 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.

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 compression 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 and 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 in _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.

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 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 described 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.

A. Side-view showing leaf-cushions on the left-hand side and the
_Knorria_ condition on the right.
B. 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 due 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).

1811. “Strobilus,” Parkinson, Organic Remains, Vol. I. p. 428, Pl.
IX. fig. 1.
1828. _Lepidostrobus ornatus_, Brongniart, Prodrome, p. 87.
1831. _L. variabilis_, Lindley and Hutton, Foss. Flora, Pls. X. XI.
1831. _L. ornatus_, Lindley and Hutton, Foss. Flora, Pl. XXVI.
1837. _L. ornatus_ var. _didymus_, _Ibid._ Pl. CLXIII.
1850. _Arancarites Cordai_, Unger, Genera et Spec. Plant. foss. p.
382.
1875. _Lepidostrobus variabilis_, Feistmantel, Palaeontographica,
Vol. LXIII. Pl. XLIV.
1886. _L. variabilis_, Kidston, Cat. Palaeozoic Plants, p. 197.
1890. _L. ornatus_, Zeiller, Flor. Valenciennes, p. 497, Pl. LXXVI.
figs. 5, 6.
—— _L. variabilis_, Zeiller, Flor. Valenciennes, p. 499, Pl.
LXXVI. figs. 3, 4.

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.

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.

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 of _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 this 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 _Lepidostrobus 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.

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.

1878. _Lepidostrobus sp_., Williamson, Phil. Trans. R. Soc., p.
340, Pl. XXII.
1880. _Lepidostrobus insignis_, Williamson, Phil. Trans. R. Soc.,
p. 502, Pl. _XV_. figs. 8–12.
1889. _Lepidodendron Spenceri_, Williamson, Phil. Trans. R. Soc.,
p. 199, Pl. VII. figs. 20–22; Pl. VIII. fig. 19.
1897. _Spencerites insignis_, Scott, Phil. Trans. p. 83, Pls.
XII–XV.

Another type of lycopodiaceous strobilus, differing sufficiently from _Lepidostrobus_ to deserve a special generic designation, is 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.

Of the two species included by Scott in his genus _Spencerites_ only one, _S. insignis_, need be considered. Since the publication 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 and 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).]