Fig. 239.—Selaginella inæqualifolia. Cone in longitudinal section; microsporangia are seen on the left side, macrosporangia on the right (most frequently each with four macrospores).
Order 1. Isoëtaceæ (Quill-worts). The only known genus, Isoëtes (Quill-wort), has an extremely short, tuberous, unbranched stem with very short internodes (Fig. [237]). The STEM is remarkable as being the only one among the Vascular Cryptogams which increases in thickness (see page [202]). The meristematic cells are situated round the axial cylinder, and form, especially, parenchymatous tissue in two or three directions, giving rise to 2–3 grooves in which the dichotomously-branched ROOTS are produced. The LEAVES are arranged spirally in a close rosette. They are awl-shaped and have at the base a semi-amplexicaul sheath, with a groove (fovea), in which a sporangium is situated (Fig. [238]). The ligule is a foliar outgrowth from the upper edge of the groove.—The MACROSPORANGIA (each with a number of macrospores), are situated on the outer leaves, the MICROSPORANGIA (Fig. [238]), on the inner ones. Between each cycle of fertile leaves there are a number of imperfect or barren ones as in the case of the female plant of Cycas. The spores are liberated by the decay of the sporangium. The two kinds of sporangia develope at the commencement in the same way. The archesporium is, at first, a hypodermal layer of cells which grow out in the direction perpendicular to the surface of the leaf, and divide by a number of walls parallel to this direction, forming a sporogenous mass of cells. Some of the cell-rows of this sporogenous mass lose their rich protoplasmic contents, and are arrested in their growth; thus incomplete divisional walls of sterile cells, “trabeculæ” arise in the sporangium, dividing it into a number of compartments one above the other (Fig. [238] t). (The trabeculæ, according to Goebel, play the same part as the nutritive cells of the sporangium of Riella; the tapetal cells, as in the Ferns, are in a great measure dissolved at a later period.) The sporogenous cell-rows, in the microsporangia, give rise to a large number of spore-mother-cells, but in the macrosporangia only one spore-mother-cell, with tapetum, is developed from each fertile archesporial cell.
The two native species, and several others, are aquatic plants, the remaining species are land plants, or are amphibious. About 50 species. In temperate and tropical regions.—Fossil species in the Tertiary period.
Order 2. Selaginellaceæ. This order contains only one genus, Selaginella. The STEM, in the majority of species, is dorsiventral, long and slender, and apparently branches dichotomously, but in reality monopodially, with well developed lateral shoots. The LEAVES are small, round, or ovate, in the majority of species arranged in whorls of two leaves each; these whorls, however, are not decussate, but are considerably inclined towards each other, an arrangement by which four rows of leaves are produced, each whorl having one large and one small leaf. The two leaves in each whorl are of unequal size, the smaller one being placed on the upper surface and the larger on the lower surface of the stem (Fig. [240]). Some species have spirally-arranged leaves, more resembling the arrangement in the Lycopodiums.
The FERTILE LEAVES most frequently differ from the barren ones, and are collected into spike-like cones (a kind of flower; Fig. [239]). Micro-and macrosporangia are found in the same cone (Fig. [239]). Each sporangium arises from a group of superficial cells of the stem, directly over the leaf on which it will be situated later on. Each sporangium has a hypodermal, unicellular archesporium, and contains a layer of tapetal cells; these are dissolved later, when the spores are ripe, and not before as in the Ferns. In the very early stages of their development, the micro-and macrosporangia are precisely similar, and the differences between them arise later on. In the microsporangium all the spore-mother-cells divide, and each forms four tetrahedrically-arranged microspores (Fig. [204]); but in the macrosporangium only four macrospores are formed, by the division of a single mother-cell, while the remaining spore-mother-cells are aborted. It is rarely that the macrosporangia contain 2 or 8 macrospores.
Fig. 240.—Selaginella martensii: s lower leaves; r upper leaves.
For the GERMINATION OF THE SPORES, see pages [228], [229]. The prothallium arises in the macrospore (f-f, in Fig. [235] A), probably by division of the meniscus-shaped protoplasmic mass, which is marked off at the apex of the spore; primordial cells are thus formed which later on are surrounded by a cell-wall. In six to seven weeks after sowing, the spore-wall is ruptured by the growing prothallium, which already has developed archegonia (Fig. [235] œ-œ). The prothallium so formed does not occupy the entire cavity of the spore, but four to five weeks after sowing, the large-celled parenchyma is developed in the lower portion of the spore by free cell-formation; this has been termed by Pfeffer, “endosperm,” since it is similar to the endosperm of Flowering-plants. Goebel, however, has termed it “secondary prothallium,” as the homology with the endosperm of the Angiosperms is very doubtful.
The FERTILISED OOSPHERE divides into an upper (hypobasal) and a lower (epibasal) cell; from the latter alone the embryo is developed with its root, stem, foot, and two cotyledons, and the former gives rise to an organ which appears in this instance for the first time, but which occurs in all Flowering-plants, viz. the suspensor. This forces the embryo down into the “endosperm,” which is entirely or partially absorbed by the embryo. In the case of the Flowering-plants the embryo is developed with its longitudinal axis in the elongation of the suspensor, but in Selaginella the embryo is situated transversely to it.