FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.
In the present day, acrogenous plants are represented by cellular and vascular Cryptogams. In considering fossil plants our attention is specially directed to the latter. In the recent Floras, vascular Acrogens are represented by such plants as Ferns, Lycopods, and Equisetums. Some of them have an arborescent habit, but the greater number are shrubby and herbaceous. Many of them have creeping rhizomes, which are either subterranean, or run along the surface of the ground. One of these arborescent forms is seen in Tree-ferns (Fig. 9). Another form with a rhizome is seen in Fig. 10. The trunks of ferns are marked by scars, which indicate the parts where the bases of the fronds were attached, and where the vascular tissue passes out from the interior (Fig. 11, _a_ and _b_). A transverse section of the stem (Fig. 12) shows a continuous cylinder of scalariform vessels (Fig. 13), enclosing a large mass of cellular tissue frequently penetrated by small scalariform bundles. The cylinder is pierced by meshes, from the inner sides of which rise the vascular bundles going to the leaves, while some of the free bundles of the axis pass through the mesh, carrying with them a portion of the cellular tissue into the petiole. The fructification consists of spore-cases (sporangia), often with an elastic ring round them, containing spores in their interior (Fig. 14).
[Sidenote: Fig. 9. Tree-fern, with a slender cylindrical trunk and a crown of drooping fronds. It is a vascular acrogen.]
[Sidenote: Fig. 10. _Asplenium_; a species of Spleenwort. A. Rhizome, _r_, covered with the bases (stalks or stipes) of the fronds; _f_, fronds in bud, rolled up in a circinate manner (this is very rarely seen in fossil ferns); _g_, fronds bearing fructification on their backs. B. Portion of a frond separated to show the linear sori or clusters of sporangia (spore-cases).]
Among Acrogens of the present day there are also plants belonging to the natural order Lycopodiaceæ or Club-mosses (Fig. 15), having creeping stems, which give rise to leafy branches. The leaves are small, sessile, and moss-like, and the fructification consists of two kinds of cellular bodies, small spores or microspores (Fig. 16), and large spores or macrospores (Fig. 17). They consist of cellular and vascular tissues, the latter occurring in the form of woody, annular, and scalariform vessels, which occupy the axis or central part of the stem. They differ from ferns in the distribution of their vascular bundles. The order is represented also by such plants as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the plant called Isoetes (Quillwort) there is a peculiar short stem which does not increase in height. It produces additions laterally, so that the stem increases in thickness. The leaves continue to multiply, and bear fructification at their bases. They have both large and small spores.