A dual system of terminology has been unavoidably adopted for species of Lepidodendron: the majority of specific names have been assigned to fossils known only in the form of casts or impressions, while petrified fragments, which unfortunately seldom show the surface-features, have received another set of names. A glance at the older palaeobotanical literature reveals the existence of several generic designations, which fuller information has shown to have been applied to lepidodendroid shoots deprived of some of their superficial tissues before fossilisation and differing considerably in appearance from the more complete branches of the same species[240]. It has in some instances been possible to correlate the two sets of specimens, casts or impressions, showing external features, and petrified fragments. We may reasonably expect that future discoveries will enable us to piece together as definite specific types specimens at present labelled with different names.
A well-preserved leaf-cushion of a Lepidodendron—the most obvious distinguishing feature of the genus—is rhomboidal or fusiform and vertically elongated ([fig. 146], C, E; [fig. 185], C, D): in exceptional cases it may reach a length of 8 cm. and a breadth of 2 cm. The cushion as a whole represents a prominent portion of the stem or branch comparable with the elevation on the twig of a Spruce Fir and the leaf-base of a Lycopodium (cf. [fig. 121], A, lower portion) which appears in a transverse section of a branch as a rounded prominence (cf. Lycopodium, [fig. 125], A and H). Disregarding differences in detail, a typical Lepidodendron leaf-cushion is characterised by a clearly defined smooth area often situated in the middle region ([fig. 146], C, s). This is the leaf-scar or place of attachment of the base of the leaf which was cut off by an absciss-layer while the branch was comparatively young, as in recent forest trees and in some species of Ferns. On the leaf-scar are three smaller scars or cicatricules, the central one is circular or more or less triangular in outline, the two lateral scars being usually oval or circular. The central pit marks the position of the single vascular bundle which constituted the conducting tissue connecting the leaf with the main vascular system of the stem. The two lateral scars (figs. [145], A, p; [146], C, s; [147], p) represent the exposed ends of two strands of tissue, the forked branches of a strand which pass from the middle cortex of the stem into the leaf; this is known as the parichnos, a name proposed by Professor Bertrand in 1891[241].
The specimen shown in [fig. 141] shows the linear leaves attached to their respective cushions.
Fig. 141. Lepidodendron Sternbergii. From a specimen in the British Museum (No. v. 1235) from the Coal-Measures of Shropshire. (Nat. size.)
The lamina has a well-defined median keel on the lower surface and on either side a groove in which sections of petrified leaves have demonstrated the occurrence of stomata (cf. [fig. 142]).
ii. Leaves and Leaf-cushions.
All Lepidodendron leaves, so far as we know, possessed a single median vein only. In some species, as for example in Lepidodendron longifolium Brongn., they have the form of long and slender acicular needles very similar to those of Pinus longifolium; in L. Sternbergii ([fig. 141]) they are much broader and shorter. In external form as in internal structure it is often impossible to distinguish between the leaves of Lepidodendron and Sigillaria. The distinguishing features enumerated by the late M. Renault cannot be employed, with any great degree of confidence, as diagnostic characters. In transverse section the lamina of a Lepidodendron leaf presents the same appearance as that of the Sigillarian leaves represented in [fig. 142]. Near the base the free part of the leaf is usually sub-rhomboidal in section with short lateral wings, a ventral keel and two stomatal grooves ([fig. 142], A, B, g). The form and arrangement of stomata are shown in [fig. 143], A, which was drawn from a piece of a leaf shown in surface-view in a section lent to me by Professor Weiss. It should, however, be pointed out that the leaf cannot be certainly identified with Lepidodendron rather than with Sigillaria, but as the leaves of these two genera are constructed on the same plan the identification is of secondary importance.
Fig. 142. Leaves of Sigillaria in transverse section.