The life of this period is thoroughly Palaeozoic and continues along the lines already marked out in the Devonian, but there are some notable changes and advances which look toward the Mesozoic order of things.
The Carboniferous vegetation is of very much the same character as that of the Devonian, but owing to the peculiar physical geography of the times, the plants were preserved as fossils in a much more complete state and in vastly larger numbers. The flora is composed entirely of the higher Cryptogams and the Gymnosperms, no plant with conspicuous flowers having come into existence, so far as we yet know. By far the most abundant of Carboniferous plants are the Ferns (Filicales) which flourished in multitudes of species and individuals, both as tall trees and as lowly, herbaceous plants. Many of these ferns cannot yet be compared with modern ones, because the organs necessary for trustworthy classification have not been recovered, and such are named in accordance with the venation of the leaves. In other cases the comparison with existing ferns may be definitely made, and these remains show that many of the modern families (Marattiacece, Ophioglossacece, etc.) had representatives in the Carboniferous forests and swamps.
Even more conspicuous, though much less varied, were the Lycopods (Lycopodiales) the remarkable character of which has been elucidated by the long-continued and laborious efforts of many investigators. While the Ferns have remained an important group of plants to the present time, the Lycopods have dwindled to a few insignificant herbaceous forms, but in Carboniferous times they were the abundant and conspicuous forest trees, at least of the swampy lowlands. One of the most characteristic of these trees was Lepidodendron (PI. X), of which many species have been found in the coal measures. These great club-mosses had trunks of 2 or 3 feet in diameter and 50 to 75 feet high, which possessed the remarkable quality, for a Cryptogam, of an annual growth in thickness. At a considerable height above the ground the trunk divides into two main branches, each of these again into two, and so on (dichotomous division). The younger parts of the tree are covered with long, narrow, stiff, and pointed leaves, while the older parts are without leaves, which have dropped off, making conspicuous scars, arranged in spiral lines around the stem.
At the ends of the twigs in some species, or on the sides of the trunk and larger branches, in others, are found the spore-bearing bodies, which have much the appearance of pine-cones. The stem was, to a large extent, filled with loose tissue and had only a relatively small amount of wood.
Plate X. - Carboniferous Vegetation.
Leptdodendron, central tree with cones. Sigillaria, each side of middle, with leafy trunks. Calamites, right side. Cordaites, left side on raised ground. Cycado/ilices, fern-like growth in foreground.
Another very characteristic and abundant tree is Sigillaria (PL X); it is closely allied to Lepidodendron, but has a very different appearance. The trunk is quite short and thick, rarely branching, and with a pointed or rounded tip, much as in the great Cactus; the leaves are similar to those of Lepidodendron, but are arranged between vertical ridges. Sigillaria also possessed the power of annual increase in diameter. Both Lepidodendron and Sigillaria are provided with branching rhizomes, or underground stems, which carry finger-like appendages inserted into pits. Before the nature of these rhizomes was understood, they were regarded as distinct plants and named Stigmaria.
A third group of Cryptogams, the Equisetates, or Horsetails, were of great importance in the Carboniferous forests. The Catamites were decidedly superior to the existing horsetails, not only in size, but in many features of organization as well. These plants had tall, slender stems divided by transverse joints, with a soft inner pith, surrounded by a ring of woody tissue, which grew annually in thickness. The shape and arrangement of the leaves differ much in the various genera, and even in different parts of the same plant; for example, they are needle-like in Astrophyllites, while in Annularia they are broad and at the base united into a ring around the stem, but some species of Annularia, at least, are probably merely the branches of larger calamites. The shape, size, and position of the spore-bearing" organs likewise differ in the different genera, but often resemble those of the modern horsetails. The base of the stem tapers abruptly, and is either connected with a horizontal rhizome or gives off a bundle of roots. Fragments of calamite stems are among the commonest fossils of the coal measures. The three preceding groups of Cryptogams all have representatives in the modern world, and one of them, the Ferns, is still abundant and varied.
In addition to these, Carboniferous vegetation had two other cryptogamic classes of great interest, which are now extinct, and are not known to have passed beyond the Palaeozoic era. Of these, the first is the class Sphenophyllales, a group of very slender, probably climbing and trailing plants, with small leaves varying in shape in different plants and different parts of the same plant. Some of the leaves, which are always small, are wedge-shaped, others are divided and others again are narrow and simple. The great interest of the class lies in the fact that it is intermediate between the horsetails and club-mosses, and doubtless its Carboniferous representatives were the survivors of an ancient group which was ancestral to both club-mosses and horsetails.
Even more remarkable is the class Cycadofilices, which was extremely abundant in the Carboniferous forests and swamps, and which affords the long-sought transition between the flower-less and the flowering plants, connecting, as its name implies, the ferns and cycads. In external appearance of stem and foliage these plants most resembled tree-ferns.
The Flowering Plants are still represented only by the Gym no-sperms, of which the dominant group is the Cordaitece (see PI. X), which were slender, very tall trees, "with trunks rising to a great height before branching, and bearing at the top a dense crown, composed of branches of various orders, on which simple leaves of large size were produced in great abundance." (D. H. Scott.) The centre of the trunk was occupied by a large soft pith, and the leaves, with their parallel venation resembling those of lilies and grasses, were long, broad in most species, narrow in others, and ' either sharply pointed or bluntly rounded. The Cordaiteae had affinities with each of the three existing orders of Gymnosperms, the Cycads, Conifers, and Gingkos, but is not referable to any of them. The three orders named may all have existed in the Carboniferous, but this is not definitely known.
The Carboniferous flora is merely the Devonian flora somewhat advanced and diversified, and the forests were of the same gloomy, monotonous character as before. The wide distribution and uniform character of this flora are very remarkable; we find the same or nearly allied species of plants spread over North America, Europe (even in the polar lands, like Spitzbergen and Nova Zembla), Siberia, China, the Sinai peninsula, Brazil, Australia, and Tasmania.