Fig. 202.—Adiantum capillus veneris. Vertical section through a prothallium (f f), with a young plant attached on its under side (mag. about 10 times); r the first root, and b the first leaf of the young Fern-plant; m the foot. In the angle between m and b lies the apex of the stem: h the rhizoids of the prothallium; æ æ unfertilised archegonia.
In the Mosses the asexual generation is the sporogonium, which is limited in its development and in a great measure dependent upon the sexual generation, upon which it is situated; but in the Pteridophyta this generation is an independent and highly developed plant, provided with stem, leaf, and true roots, and has in many instances an unlimited development. The Pteridophyta are the lowest Division with true roots. The root which is first formed is very similar in nature to the primary root of the Monocotyledons; it very soon dies and is replaced by others which are more permanent, and developed upon the stem (adventitious roots); roots are wanting in Salvinia, Psilotum, and some Hymenophyllaceæ. The differentiation is, however, not so complete as in the Flowering-plants, and so many leafy forms are not found. The various members of these plants are anatomically much higher than in the Mosses, having an epidermis, a ground tissue with variously differentiated cells, and a highly developed vascular system. The vascular bundles, like those in the Monocotyledons, are without cambium, and closed; they are therefore incapable of any increase in thickness. In general the bundles are concentric, with the bast round the wood (Fig. [203]). The wood is almost entirely made up of scalariform tracheides.
In Isoëtes a secondary thickening takes place by a cambium, which is formed inside the cortex, constructing secondary cortex to the exterior, and secondary wood towards the interior.—Botrychium has also a thickening growth. Collateral vascular bundles occur in Osmundaceæ, Equisetaceæ, and the leaves of many Polypodiaceæ, etc.
Fig. 203.—Portion of the stem of a Fern. Above is seen the transverse section, with vascular bundles of different form and size. The rhombic figures on the side of the stem are leaf-scars.
It is a point of special interest, that the gigantic forms of Ferns, Equisetums, and Club-Mosses (which flourished in earlier geological periods, when these classes attained their highest development) possessed some means of increasing in thickness.
The sporangia are in all cases capsule-like, and burst open when ripe to eject the spores. They are nearly always situated on the leaves (in Lycopodiaceæ, in the axils of the leaves, or above these, on the stems themselves). In some forms (Leptosporangiatæ), the sporangia are developed from a single epidermal cell; in others (Eusporangiatæ), from a group of epidermal cells, or from cells which lie beneath the epidermis. In the first group a primitive mother-cell (archesporium) is formed, which divides commonly into sixteen special mother-cells. In the latter group, on the other hand, a number of primitive spore-mother-cells are developed. In each sporangium three different tissues are generally developed; an innermost sporogenous one (s in Fig. [204] A), which arises from the archesporangium; an outermost one, which forms the wall (a), and may be one or, more rarely, several layers in thickness; and an intermediate one, the tapetum (Fig. [204] A, B, b t), which is rich in protoplasm, and whose cells are dissolved so that the spores float freely in the fluid thus provided. The spores arise as in the Mosses (in tetrads), by the cross-division of the special mother-cells, and according to the manner in which they are arranged in the mother-cell have either a tetrahedral form, with a large base resembling a segment of a ball, or are oblong (bilateral spores). Their construction is the same as in the Mosses (p. [187]).
Fig. 204.—Selaginella inæqualifolia. A A young sporangium, which may develope either into a macro-, or a microsporangium. B A microsporangium.
The spore-formation in its earliest commencement takes place in the same way in the Isosporous and the Heterosporous Vascular Cryptogams; but from a certain point, after the tetrahedral division, a difference occurs with regard to the macrosporangia. All the spores formed in the microsporangium may complete their development; but those which are formed in the macrosporangium are generally aborted, with the exception of one or four, and these consequently attain a much larger size (see Fig. [239].—The series to the left are microsporangia; those to the right, macrosporangia).