This species was founded by Renault on petrified specimens from the Culm beds of Esnost in France. The surface of a young twig bears prominent leaf-cushions of elongated rhomboidal form similar to those of Lepidodendron obovatum ([fig. 173]) and other species. In older branches the primary cortex is replaced by a considerable thickness of radially disposed secondary cortical tissue which, as shown in tangential section, consists of a reticulum of elongated pointed elements with comparatively thick walls enclosing meshes filled with large-celled parenchyma. It is worthy of note that if such a branch were exposed to decay, the earlier destruction of the more delicate tissue in the meshes of the secondary cortex would produce a series of oval depressions, corresponding to the parenchymatous areas, separated by a projecting reticulum of the more resistant elements: a cast of this partially decayed surface would be indistinguishable from that of some types of Sigillaria or of a Lyginodendron. The inner regions of the cortex of the type-specimens have not been preserved. The xylem, which is the only part of the stele represented, has the form of a protostele or solid cylinder of scalariform tracheids with peripheral groups of narrower protoxylem elements which mark the points of exit of the leaf-traces: in a branch 1–2 cm. wide the xylem column has a diameter of 3 mm. The small leaves ([fig. 143], B, C), similar to those of a Sigillaria, are sub-rhomboidal in section near the base and approximately circular near the apex[320]. The mesophyll consists of palisade cells having the appearance of typical chlorophyll-tissue. The heterosporous strobili attributed to this species bore microsporangia on the upper and megasporangia on the lower sporophylls; the megaspores, of which a considerable number occur in each megasporangium, are identical in size with those of another Culm form, Lepidodendron rhodumnense. Some of these have retained traces of prothallus tissue, and in one spore Renault figures what he regards as an archegonium: the drawing is by no means convincing.
2. Lepidodendron rhodumnense, Renault[321].
The species from the Culm of Combres (Loire) agrees in its solid xylem cylinder and in the differentiation of the secondary cortex, as also in the association of two kinds of spore, with Lepidodendron esnostense. A comparison of the leaves of the two types reveals certain differences which may be of specific rank, but, apart from minor differences, these Culm species may be classed under one anatomical type.
3. Lepidodendron saalfeldense, Solms-Laubach[322].
This Devonian species was founded on a specimen 3 × 2·5 cm. broad at the base, which shows the stumps of four branches recalling the dichotomously branched arms of Stigmaria and Pleuromeia. If these are in reality the remains of Stigmaria-like horizontal branches the species affords an interesting example of a Lepidodendron axis with a subterranean rhizome of the type which has been found in several Sigillarian stems. In the upper end of the axis the stele consists of a solid strand of xylem which is not sufficiently well preserved to show the position of the protoxylem groups. A transverse section taken near the base reveals a type of stele differing from that at the upper end in being composed of radially disposed tracheids and in its resemblance to the stele of Stigmaria.
4. Lepidodendron fuliginosum, Williamson. Figs. [162–172], [179], E.
- 1871. Lepidodendron Harcourtii, Binney, Palæont. Soc., p. 48, Pl. VII. fig. 6.
- 1872. Halonia regularis, Binney, Palæont. Soc., p. 89, Pl. XV.
- 1881. Lepidodendron Harcourtii, Williamson, Phil. Trans. Roy. Soc., Vol. 172, p. 288, Pls. XLIX–LII.
- 1887. Lepidodendron fuliginosum, Williamson, Proc. Roy. Soc., Vol. XLII. p. 6.
- 1891. Lepidodendron Williamsoni, Solms-Laubach, Fossil Botany, p. 226.
- 1893. Lepidophloios fuliginosus, Kidston, Trans. Roy. Soc. Edinburgh, Vol. XXXVIII. p. 548.
The name Lepidodendron fuliginosum was proposed by Williamson in 1887 for petrified stems previously included by him in Witham’s species L. Harcourtii, but subsequently recognised as a distinct type characterised by “the greater uniformity in the composition of the entire cortex” and by other features some of which do not constitute distinctive characters. The species agrees with L. Harcourtii and with L. Veltheimianum in having a medullated stele; it is distinguished not only by the more frequent preservation of the middle cortex, a fact due to a difference in minute structure, but chiefly by the peculiar structure of the secondary tissue added to the stele; this is in part composed of radial series of parenchymatous cells and of a varying amount of tracheal tissue the elements of which are narrower than in other species and are characterised also by their sinuous vertical course. As is pointed out in the sequel, the anatomical features of L. fuliginosum, as at present understood, are not confined to one type of Lepidodendron stem. Specimens have been described with leaf-cushions of the form characteristic of L. aculeatum, L. obovatum and Lepidophloios combined with the anatomical features of Williamson’s species: it is possible that the two species L. obovatum and L. aculeatum are not really distinct[323], but it is certain that shoots with both the Lepidodendron and Lepidophloios cushions may have the same type of anatomical structure.
A more detailed knowledge of the structural features of Lepidodendron shoots may enable us to define anatomical species with more exactness than is possible at present. There can, however, be little doubt that well-marked anatomical features may be associated with more than one specific form of shoot as defined by the form of the leaf-cushions.
Solms-Laubach proposed the name Lepidodendron Williamsoni for the anatomical type L. fuliginosum of Williamson, but the latter name has been generally adopted.