Fig. 9. Tree-fern, with a slender cylindrical trunk and a crown of drooping fronds. It is a vascular acrogen.
Fig. 10.
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.
Fig. 11, a. Fig. 11, b.Fig. 12.
Fig. 11, a. Bifurcating (forked or dichotomous) trunk (caudex) of a Tree-fern (Alsophila Perrottetiana), showing the scars (cicatrices) left by the fallen fronds. These scars exhibit the arrangement of the vascular bundles. Fig. 11, b. Rhizome of Lastrea Filix-mas (male fern), showing scars of the leaves, c, with markings of the vascular bundles.
Fig. 12. Transverse section of the stem (caudex) of a Tree-fern (Cyathea), showing the arrangement of the cellular and vascular tissue. The cellular tissue of the centre, m; that of the circumference, p; vascular cylinder, f v, consisting of dark-coloured pleurenchyma or ligneous tubes, f, and paler vessels, v, chiefly scalariform and closed spiral, and pierced by the meshes for the leaf-bundles at m; the outer cortical portion connected with the bases of the leaves, e.
