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. [Illustration: FIG. 142. Leaves of _Sigillaria_ in transverse section. A, A′. Section in the Manchester University Museum (Q. 631). B, C. Sections in Dr Kidston’s Collection.] The single xylem bundle consists of primary tracheae only, at least in such laminae as have been identified as Lepidodendroid. Surrounding the xylem strand occur delicate parenchymatous cells in some cases accompanied by darker and thicker-walled elements. As in _Sigillaria_, the leaves of which are more fully described on page 210, a fairly broad sheath of wider and shorter scalariform or spiral transfusion tracheids surrounds the conducting strand (figs. 142, _t_; 143, B, C, _t_). As Renault shows in the case of _Lepidodendron esnostense_[242], the small leaves of which are 1·5–2 mm. broad at the base and several centimetres long, the stomatal grooves and keel die out towards the apex when the lamina assumes a more nearly circular form (fig. 143, C). [Illustration: FIG. 143. A. Stomata in surface-view (_Lepidodendron_?). _a_, parenchyma; _t_, transfusion tracheae; _x_, xylem. (Manchester University Collection R. 723). B, C. _Lepidodendron esnostense_ Ren. (After Renault.)]