[Illustration: Fig. 48. Polytrichum commune:—group of antheridia mixed with hairs and sterile filaments (paraphyses), the central one discharging its contents.]
[Illustration: Fig. 49. Polytrichum commune:—B, cellular contents of an antheridium previously to the development of the spermatozoids; C, the same, showing the first appearance of the spermatozoids; D, the same, mature and discharging the spermatozoids.]
The urn-shaped organ or sporangium has a double wall, and in its centre a fusiform or deeply winged columella. Between these two there is a tissue of a most delicate texture, divided into spherical cells, each of which usually produces four unripe spores, which get an exterior coat when fertilised, and then it is that the stalk lengthens, and the flask-shaped archegonium is torn across.
The urns are closed by a lid or operculum of a flat, convex, or pointed form, which falls off when the spores are ripe to give them egress. The mouth of the urn in most of the mosses is surrounded by a deciduous annulus of two or three rows of elastic cells, which are supposed to aid in scattering the spores, for the mosses have no elaters. Although the separation of the lid may at once expose the spores, they may be covered with a membrane entire or toothed at the circumference, or there may be one or two rows of teeth surrounding the mouth of the urn like a fringe; these teeth form the peristome, and their number is always a multiple of 4, varying from 4 to 64 (see fig. 47 _p_). Sometimes they are divided half way down, sometimes they are prolonged into straight twisted hairs. The teeth arise from the thickening of the walls of two contiguous cells; when there are two rows of teeth, the outer row frequently arises from the layer of cells which line the outer wall of the urn, the inner row from the outermost layer of the spore sac; sometimes, when the peristome is double, three strata of cells are requisite to form the teeth. The urns of the Encalypta vulgaris are represented at fig. 47 B; one is covered with the hood or calyptra, while from the other the lid has fallen off, showing the mouth of the urn with its toothed peristome.
When a spore begins to grow, its outer coat is ruptured, its innermost coats protrude and form a projecting extremity, which becomes developed into a confervoid pro-embryo, analogous to the mycelium of fungi, but is distinguished by the chlorophyll contained in its cells. Many spores may concur or not in its formation; but the pro-embryo of each spore is capable of transforming one or more of the cells seated upon its various ramifications immediately into buds, which grow up into leafy stems, as the Funaria hygrometrica, so that here we have the peculiar condition of one spore giving rise to the development of a number of plants.[68] In process of growth each of these plants would produce antheridia and archegonia, and, by the process described, a full-grown Funaria hygrometrica with its urns and hoods, as represented in fig. 47 A, would be the result.