The family of the Club-mosses, already, even in the Devonian, in advance of its modern development, experiences in the Carboniferous a remarkable and portentous extension into great trees of several genera and many species, constituting apparently extensive forests, and having the woody tissues of their stems developed to a degree unheard of in their present representatives (Fig. 99). Further, they become closely linked, in external form at least, with another and more advanced type, that of the _Sigillariæ_. These remarkable trees were the most abundant of all in the swamps of the coal-formation, and probably those which most contributed to the accumulation of coal. They presented tall pillar-like trunks, often ribbed longitudinally, and with perpendicular rows of scars of fallen leaves. Dividing at top into a few thick branches, they were covered with long rigid grass-like foliage. Their fruit was borne in rings or whorls of spikes surrounding the branches at intervals (Fig. 100). Their roots were strangely symmetrical, spreading out like underground branches into the soft soil by a regular process of bifurcation, and were covered with rootlets diverging in every direction, and so jointed to the main root that when broken off they left round marks regularly arranged. These roots are the so-called _Stigmariæ_, so abundant in every coal-field, and especially filling the “under-clays” of the coal-beds, which are the soils on which the plants forming these beds were supported. The true botanical position of the _Sigillariæ_ has been a matter of much controversy. Some of them undoubtedly have structures akin to those of the tree-like Club-mosses, as Williamson has well shown, and may have been cryptogamous. Others have structures of higher character, akin to those of the modern Cycads, and seem to have borne nutlets allied to those of these plants. Yet the external forms of these diverse sorts are so similar that no definite separation of them has yet been made. Either these anomalous trees constitute a link connecting the two great series of the vegetable kingdom, or we have been confounding two distinct groups, owing to imperfect information. [Illustration: FIG. 99.—_Lepidodendron corrugatum_ (Dn.). A characteristic Lycopod of the Lower Carboniferous of America. A, Restoration. B, Leaf, natural size. C, Cone. D, Leafy branch. E, Forms of leaf-bases. F, Sporangium. I, L, M, N, O, Markings on stem and branches, in various states.] [Illustration: FIG. 100.—_Sigillariæ_ of the Carboniferous. A, _Sigillaria Brownii_ (Dn.). B, _S. elegans_ (Brongniart). B¹, &c. Leaf and Leaf-scars.]