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.
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.
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.
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.
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.
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.
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.
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].
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.
Spencerites insignis (Williamson). Fig. 192.
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).]