The markings left on the well-preserved leaf-scars indicate the main features of the internal anatomy of the leaves. They had a single central vascular strand (_v_, fig. 93), on either side of which ran a strand of soft tissue _p_ called the parichnos, which is characteristic of the plants of this group. While another similarly obscure structure associated with the leaf is the little scale-like ligule _l_ on its upper surface. The anatomy of the stems is interesting, for in the different species different stages of advance are to be found, from the simple solid protostele with a uniform mass of wood to hollow ring steles with a pith. An interesting intermediate stage between these two is found in _Lepidodendron selaginoides_ (see fig. 95), where the central cells of the wood are not true water-conducting cells, but short irregular water-storage tracheides (see p. 56), which are mixed with parenchyma. All the genera of these fossils have a single central stele, round which it is usual to find a zone of secondary wood of greater or less extent according to the age of the plant. [Illustration: Fig. 95.—Transverse Section of _Lepidodendron selaginoides_, showing the circular mass of primary wood, the central cells of which are irregular water-storage tracheides _s_, Zone of secondary wood; _c_, inner cortical tissues; _r_, intrusive burrowing rootlet; _oc_, outer cortical tissues with corky external layers _k_. (Microphoto.)] Some stems instead of this compact central stele have a ring of wood with an extensive pith. Such a type is illustrated in fig. 96, which shows but a part of the circle of wood, and the zone of the secondary wood outside it, which greatly exceeds the primary mass in thickness. This zone of secondary wood became very extensive in old stems, for, as will be imagined, the primary wood was not sufficient to supply the large trunks. The method of its development from a normal cambium in radiating rows of uniform tracheides is quite similar to that which is found in the pines to-day. This is the most important difference between the living and the fossil stems of the family, for no living plants of the family have such secondary wood. On the other hand, the individual elements of this wood are different from those of the higher families hitherto considered, and have narrow slit-like pits separated by bands of thickening on the longitudinal walls. Such tracheides are found commonly in the Pteridophytes, both living and fossil. Their type is seen in fig. 96, B, which should be compared with that in figs. 78, A and 62, B to see the contrast with the higher groups. [Illustration: Fig. 96.—A, _Lepidodendron_ Stem with Hollow Ring of Wood W and Zone of Secondary Wood S. B, Longitudinal View of the Narrow Pits of the Wood Elements.]