Fig. 147, A, affords a good example of a tangential section through a Lepidophloios leaf-cushion, 1 cm. broad, like that represented in [fig. 146], A, showing the vascular bundle lt, the two parichnos strands, p, composed of large thin-walled cells (cf. Isoetes, [fig. 133], H, I), and the ligular pit near the upper edge of the section enclosing the shrunken remains of the ligule ([fig. 147], B, l).
LEPIDODENDRON
[Fig. 147], B, shows the form of the tangentially elongated leaf-cushions of Lepidophloios and their spiral disposition.
[Fig. 146], F, represents a section similar to that shown in [figs. 147], A and B, but in this case the leaf-trace, lt, and the parichnos strands, p, lie in a cavity formed by the destruction of some of the leaf-cushion tissue. It is worthy of notice that the parichnos cells have resisted decay more successfully than the adjacent tissue of the cushion.
The diagrammatic sketches reproduced in [fig. 146], H and I, were made from a transverse section similar to one originally figured by Williamson[259]: [fig. 146], H, corresponding in position to the line gh in [fig. 146], A, passes through the ligular pit, l, and cuts across the parichnos in the act of branching; the leaf-trace passes outwards beyond the Y-shaped parichnos strand. In the other section, [fig. 146], I, the parichnos is shown in a horizontal plane and the leaf-trace, lt, appears in oblique transverse section. In both sections and in [fig. 146], G the shaded band at the base represents the secondary cortical tissue external to the phellogen.
The transverse section represented in [fig. 146], G, shows in the left-hand cushion, a, the exit of the two parichnos arms and the leaf-trace between them: it illustrates also the various forms assumed by lepidodendroid leaf-cushions when cut across at different levels.
iv. The Anatomy of Lepidodendron vasculare Binney[260].
[Figs. 148–155], [168], A.
In the earlier literature dealing with the anatomy of Lepidodendron and Sigillaria the presence or absence of secondary vascular tissue was made the criterion of generic distinction and the distinguishing feature between the classes Pteridophytes and Gymnosperms, Lepidodendron being relegated to the former class because it was supposed to have no power of forming secondary wood, while Sigillaria, characterised by a considerable development of such tissue, was classed by Brongniart and afterwards by Renault as a Gymnosperm. Binney[261] in 1865 recognised that the two types of stem pass into one another, but it was Williamson[262] who provided complete demonstration of the fallacy of the Brongniartian view.
These two undoubted Pteridophytes agree very closely in anatomical structure and both are now recognised as arborescent genera of Lycopodiaceous plants. In a paper published by Lomax and Weiss in 1905[263] a specimen is described from the Coal-Measures of Huddersfield, in which a decorticated stem with the anatomical characters of Binney’s Sigillaria vascularis gives off a branch having the anatomical structure which it has been customary to associate with the species Lepidodendron selaginoides, so-called by Sternberg and founded by him on impressions showing well-preserved external characters.
In 1862 Binney[264] described petrified specimens of vegetative shoots from the Lower Coal-Measures of Lancashire under the names Sigillaria vascularis and Lepidodendron vasculare. These were afterwards recognised as different states of the same species. A few years after the publication of Binney’s paper Carruthers[265] identified Binney’s species Lepidodendron vasculare with Sternberg’s L. selaginoides. The evidence on which this identification rests has not been stated, but many writers have retained this specific designation for the well-defined type of anatomical structure first described by Binney as L. vasculare. The use of the specific name selaginoides is, however, open to objection. The species Lepidodendron selaginoides, as pointed out by Kidston[266], is probably identical with the plant which Brongniart had named L. Sternbergii before the institution of Sternberg’s species, and we are not in possession of convincing evidence as to the connection of L. Sternbergii (= L. selaginoides) with specimens possessing the anatomy of Binney’s type. Binney’s designation is therefore retained for the anatomical type described in the following pages[267].