[Sidenote: LEPIDODENDRON.] Lepidodendron (_scaly-tree_). _Lign. 39._--Stems cylindrical, covered towards their extremities with simple, linear, or lanceolate leaves, which are attached to elevated rhomboidal spaces, or papillæ; papillæ marked in the upper part with a large transverse triangular scar; lower part of the stem destitute of leaves. [Illustration: Lign. 39. Fig. 1.--A terminal Branch of a Lepidodendron; _nat._ 2.--Leaf-scars on the stem of a Lepidodendron; _nat._ _In Coal-shale, Newcastle._] The remains of this tribe of plants abound in the coal formation, and rival in number and magnitude the Calamites and Sigillariæ previously described. These trees have received the name of _Lepidodendra_, from the scaly character of their stems, occasioned by the angular scars left by the separation of the foliage, as is the case in arborescent ferns: the term, however, simply indicates the appearance, for the surface is not imbricated. Some of these trees have been found almost entire, from the roots to their topmost branches. One specimen, forty feet high, and thirteen feet in diameter at the base, divided towards the summit into fifteen or twenty branches, was discovered in the Jarrow coal-mine.[105] The foliage consists of simple, linear leaves, spirally arranged around the stem, and appears to have been shed from the base of the tree with age. The scars produced by the attachment of the petioles were persistent; and the twigs and branches are generally found covered with foliage, as in _Lign. 39_. The roots are Stigmariæ, like those of the Sigillariæ, as proved by specimens in the Pictou coal-field, discovered by Mr. Brown.[106] [105] Wond, p. 722. This specimen is figured and described in Foss. Flor. [106] Petrifactions, p. 39. The internal organization of the stem of Lepidodendron differs from that of Sigillaria, in the absence of the woody cylinder and medullary rays which constitute so peculiar and important a character in the latter. The Lepidodendra have only an eccentric, vascular, medullary zone, the interval between which and the bark is filled up by cellular tissue.[107] In their structure, external configuration, mode of ramification, and disposition of the leaves, they accord so closely with the _Lycopodiaceæ_, that, notwithstanding the disparity in size, M. Brongniart, Dr. Joseph Hooker, and other eminent botanists, concur in regarding them as gigantic arborescent club-mosses[108] The living species of Lycopodiaceæ amount to nearly two hundred, the greater number of which, like the arborescent ferns, inhabit the islands of intertropical regions. They are diminutive plants, with delicate foliage, none exceeding three feet in height; most of them trail on the ground, but there are a few erect species, one of which (_Lycopodium densum_) is abundant in New Zealand. [107] See M. Ad. Brongniart, Archives du Museum d'Hist. Nat. torn. i. (for 1839), pl. XXX.