The relation of the parichnos to the oval scars below a Lepidodendron leaf-cushion has been worked out in detail by Weiss who shows that, at least in some species, the two arms do not bend downwards as shown in the diagram, fig. 144, B, but pursue a straight gradually ascending course as seen in fig. 145, A. Just below the leaf-scar region of the cushion each arm comes into association with a group of lacunar, aerenchymatous tissue, such as occurs in the roots of certain Mangrove plants, and it is this aerenchyma which is exposed on the two oval depressions below the leaf-scar. The structure of this aerenchyma is shown in fig. 145, B; it consists in this species (_L. Hickii_ Wats.) of stellate cells which would constitute an efficient aerating system. Probably, as Weiss suggests, these patches of aerenchyma were originally covered by an epidermis provided with stomata, and it is owing to the destruction of this superficial layer that the two oval scars often form a prominent feature on Lepidodendron leaf-bases[244]. The diagram reproduced in fig. 144, B, may be taken as practically correct, as the patches of aerenchyma described by Weiss do not differ essentially from the parichnos tissue. [Illustration: FIG. 144. _Lepidodendron Veltheimianum_ Sternb. A. Leaf-cushion and leaf-scar seen in surface-view at _a_; on the rest of the specimen a slightly lower surface is exposed. (After Stur.) B. Diagrammatic longitudinal section to explain the differences between its two surfaces _a_ and _b_ shown in fig. A. The shaded portion _c_ represents the rock matrix, the surfaces _ab_, _ed_, mark the outer and inner edge of the outer portion of the bark of the Lepidodendron stem. _lt_, leaf-trace; _p_, _p′_, parichnos.] [Illustration: FIG. 145. A. Diagrammatic surface-view and longitudinal section of a Lepidodendron leaf-cushion. B. Aerenchyma below the leaf-scar. (After F. E. Weiss.)]