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.
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.
Fig. 100.—Sigillariæ of the Carboniferous.
A, Sigillaria Brownii (Dn.). B, S. elegans (Brongniart). B1, &c. Leaf and Leaf-scars.
Another curious, and till recently little understood, group of Carboniferous trees is that known as Cordaites, which existed already in some of its species in the Devonian. Their leaves are long, and often broad as well, and with numerous delicate parallel veins, resembling in this the leaves of grasses. Corda long ago showed that one species at least has a stem allied to the Club-mosses. More recently Grand’ Eury has found in the South of France admirably preserved specimens, which show that others more resembled in their structure the Pines and Yews, and were probably Gymnosperms, approaching to the Pines, but with very peculiar and exceptional foliage, of which the only modern examples are the broad-leaved Pines of the genus Dammara (Frontispiece to Chapter). Here again we have either two very distinct groups, combined through our ignorance, or a connecting link between the Lycopods and the Pines.