Fig. 173. Lepidodendron obovatum. (From a specimen lent by Dr D. H. Scott.)
Dr Scott[332] published in 1906 a short account of the structure of a specimen from the Lower Coal-Measures of Lancashire, the external features of which were identified by Kidston with those of Lepidodendron obovatum Sternb. Dr Scott generously allowed me to have drawings made from his specimen; these are reproduced in [fig. 173]. The form of the leaf-cushion is by no means perfect; there is a well-marked median ridge, and the small circular scar near the upper end of some of the cushions may represent the ligular cavity. At the base of the leaf-cushions a cortical meristem has produced a zone of secondary cortex; at c a second meristem is seen in the outer cortex: the dark dots in the cortex mark the positions of leaf-trace bundles. The inner cortex, d, is a more compact tissue surrounding the imperfectly preserved secretory zone. From the medullated stele a lateral branch, b, is being given off; its crescentic form becoming changed to circular as it passes nearer to the surface.
Fig. 174. Lepidodendron aculeatum. (Cambridge Botany School.)
Fig. 175. Lepidodendron aculeatum. (Cambridge Botany School.)
A type of Lepidodendron, L. Hickii, founded on anatomical characters by Mr Watson[333], is believed by him to possess leaf-cushions like those of L. obovatum; if this is so, it is interesting, as he points out, to find two distinct anatomical types associated with one species. Watson thinks it probable that the “species” L. obovatum includes at least two widely different species. This merely emphasizes the importance of correlating structure and external characters as far as available data permit.
Fig. 176. Lepidodendron aculeatum. (Cambridge Botany School.)
The specimen, of which part of the surface is shown in [fig. 174], is in all probability L. aculeatum Sternb. This was described by me in detail in The Annals of Botany (1906) as another example of the co-existence of the Lepidodendron fuliginosum type of anatomy with a true Lepidodendron. The locality of the specimen is not known. The leaf-cushions are 1·5 cm. long with tapered upper and lower ends; a ligular cavity may be recognised on some parts of the fossil, also faint indications of leaf-trace scars. The tubercles ([fig. 174], A–C, t) probably represent leaf-traces which the shrinkage of the superficial tissues has rendered visible in the lower part of their course. The circular scar, s (fig. B), on the partially decorticated surface is apparently a wound. The stele is sufficiently well preserved to justify its reference to L. fuliginosum. The irregularly crenulated edge of the primary xylem, x ([fig. 175]), is succeeded by a broad band of parenchyma (the meristematic zone), m, and beyond this are remnants of the secretory zone, s. The structure of the leaf-traces corresponds with that of other specimens of the type, but the much steeper course of these vascular strands, lt, lt′ ([fig. 176]), is a feature in which this example differs from most of those referred to L. fuliginosum. Such evidence as is available would seem to point to the absence of trustworthy criteria enabling us to separate, on anatomical grounds, Lepidophloios and Lepidodendron[334].