For some time the embryo has a nearly cylindrical form, but as it approaches maturity the differentiation into stalk and capsule becomes apparent. The latter increases rapidly in diameter, assuming gradually the oval shape of the full-grown capsule. A longitudinal section of the nearly ripe capsule ([Fig. 58], G) shows two distinct portions; an outer wall of two layers of cells, and an inner mass of cells in some of which the spores are produced. This inner mass of cells is continuous with the upper part of the capsule, but connected with the side walls and bottom by means of slender, branching filaments of chlorophyll-bearing cells.
The spores arise from a single layer of cells near the outside of the inner mass of cells (G, sp.). These cells (H, sp.) are filled with glistening, granular protoplasm; have a large and distinct nucleus, and no chlorophyll. They finally become entirely separated and each one gives rise to four spores which closely resemble those of the liverworts but are smaller.
Near the base of the capsule, on the outside, are formed breathing pores ([Fig. 58], F) quite similar to those of the higher plants.
If the spores are kept in water for a few days they will germinate, bursting the outer brown coat, and the contents protruding through the opening surrounded by the colorless inner spore membrane. The protuberance grows rapidly in length and soon becomes separated from the body of the spore by a wall, and lengthening, more and more, gives rise to a green filament like those we found attached to the base of the full-grown plant, and like those giving rise to buds that develop into leafy plants.
Classification of the Mosses.
The mosses may be divided into four orders: I. The peat mosses (Sphagnaceæ); II. Andreæaceæ; III. Phascaceæ; IV. The common mosses (Bryaceæ).
Fig. 64.—A, a peat moss (Sphagnum), × ½. B, a sporogonium of the same, × 3. C, a portion of a leaf, × 150. The narrow, chlorophyll-bearing cells form meshes, enclosing the large, colorless empty cells, whose walls are marked with thickened bars, and contain round openings (o).
The peat mosses ([Fig. 64]) are large pale-green mosses, growing often in enormous masses, forming the foundation of peat-bogs. They are of a peculiar spongy texture, very light when dry, and capable of absorbing a great amount of water. They branch ([Fig. 64], A), the branches being closely crowded at the top, where the stems continue to grow, dying away below.
Fig. 65.—Forms of mosses. A, plant of Phascum, × 3. B, fruiting plant of Atrichum, × 2. C, young capsule of hairy-cap moss (Polytrichum), covered by the large, hairy calyptra. D, capsules of Bartramia: i, with; ii, without the calyptra. E, upper part of a male plant of Atrichum, showing the flower, × 2. F, a male plant of Mnium, × 4. G, pine-tree moss (Clemacium), × 1. H, Hypnum, × 1. I, ripe capsules of hairy-cap moss: i, with; ii, without calyptra.