The generic name Lepidophloios was first used by Sternberg[254] for a Carboniferous species which he had previously described as Lepidodendron laricinum. In 1845 Corda[255] instituted the name Lomatophloios for specimens possessing the same external characters as those for which Sternberg had chosen the name Lepidophloios. The leaf-cushions of Lepidophloios differ from those of the true Lepidodendron in their relatively greater lateral extension (cf. [fig. 146], A and C), in their imbricate arrangement and in bearing the leaf, or leaf-scar, at the summit. In some species referred to Lepidophloios the cushions are however vertically elongated and in this respect similar to those of Lepidodendron: an example of this type is afforded by Lepidophloios Dessorti a French species described by Zeiller[256]. In younger branches the cushions may be directed upwards having the leaf-scar at the top; but in the majority of specimens the cushions are deflexed as in figs. [146], D; [160], A. The shoot of Lycopodium dichotomum shown in [fig. 121], B, with the leaves in the reversed position bears a close resemblance to a branch of Lepidophloios.
The photograph of Lepidophloios scoticus Kidst.[257] reproduced in [fig. 160], A, illustrates the dichotomous branching of the stem and the form of the cushions with the leaf-scars pointing downwards. In the fertile branch of the same species shown in [fig. 160], B, the leaf-scars face upwards.
In most species the cushions are simply convex without a median keel, but in some cases a median ridge divides the cushion into two cheeks as in the genus Lepidodendron. The leaf-scar bears three small scars, the larger median scar marking the position of the leaf-trace, while the lateral scars are formed by the two arms of the parichnos: in some examples of deflexed cushions, though not in all, a ligular pit occurs on the cushion a short distance above the leaf-scar.
The drawing reproduced in [fig. 146], A, showing the leaf-scar on the upper edge of the cushion should have been reversed with the leaf-scars pointing downwards. This figure represents part of the surface of a specimen consisting of the outer cortex of a stem with leaf-cushions 3 cm. broad. The thickness of this specimen is 4 cm.: a section through the line ab is represented in [fig. 146], D (reproduced in the correct position, with the leaf-scars, sc, pointing downwards): internal to the cushions is a band of secondary cortex (the shaded strip on the outer edge of the section) which was formed on the outside of the phellogen. The phellogen is a cylinder of actively dividing cells in the outer part of the cortex of the stem, often spoken of as the cork-cambium or cortical meristem, which produces a considerable amount of secondary cortical tissue on its inner face and a much smaller amount towards the stem surface. This delicate cylinder frequently forms a natural line of separation between the outer shell of bark and the rest of the stem. In the specimen before us, the thin-walled cells of the phellogen were ruptured before petrification and the outer shell of bark was thus separated as a hollow cylinder from the rest of the stem: this cylinder was then flattened, the two inner surfaces coming into contact. Fig. 146, D, represents a section of one half of the thickness of the flattened shell.
This separation of the outer cortex, and its preservation apart from the rest of the stem, is of frequent occurrence in fossil lycopodiaceous stems. The flattened outer cortical shell of a Lepidophloios, specifically identical with that shown in [fig. 146], A and D, was erroneously described by Dr C. E. Weiss in 1881 as a large lepidodendroid cone[258].
Fig. 146, B, affords a view of the inner face of the specimen of which the outer surface is seen in [fig. 146], A: the surface shown in the lower part of the drawing, on which the boundaries of the cushions are represented by a reticulum, corresponds to the inner edge of the strip of secondary cortical tissue represented by the vertically shaded band in [fig. 146], D.
The shaded surface in [fig. 146], B, represents a slightly deeper level in the stem which corresponds to the outer edge of the vertically shaded band of [fig. 146], D: the narrow tapered ridges ([fig. 146], B) represent the leaf-traces passing through the secondary cortex, and the fine vertical shading indicates the elongated elements of which this strip of secondary cortex is composed.
In the longitudinal section diagrammatically reproduced in [fig. 146], D, cut along the line ab of [fig. 146], A, the parenchymatous tissue of the stout cushions has been partially destroyed, as at a; at s is seen the section of a Stigmarian rootlet which has found its way into the interior of a cushion. Each leaf-trace is accompanied by a parichnos strand as in the true Lepidodendron; at the base of the leaf-cushion the parichnos branches into two arms which diverge slightly right and left of the leaf-trace, finally entering the base of the leaf lamina as two lateral strands ([fig. 147], p). At one point in [fig. 146], D the section has shaved a leaf-trace represented by a black patch resting on the parichnos just above the line ef, but it passes through one of the parichnos arms p′ which debouches on to the leaf-scar sc at p. Had the section been cut along the line cd of [fig. 146], A the leaf-trace would have been seen in a position similar to that occupied by the parichnos p′ in [fig. 146], D.
Fig. 147. Lepidophloios leaf-cushion in tangential section. (From a section in the Williamson Collection, British Museum, No. 1973.)