Fig. 191.—Andreæa rupestris. Transverse section through a ripe sporangium. In the middle is seen the four-sided columella, surrounded by the numerous spores, drawn diagrammatically. Surrounding them is seen the wall of the sporangium, whose outer layer of cells is thickened and coloured. The layer of cells is unthickened in four places (x), indicating the position of the clefts (see Fig. [193]).
Among the sexual organs, paraphyses—filamentous or club-shaped bodies—are to be found.
The asexual generation, the sporophyte (Moss-fruit or sporogonium). As the result of fertilisation the oosphere surrounds itself with a cell-wall, and then commences to divide in accordance with definite laws.[17] The embryo (Fig. [189] C) produced by these divisions remains inside the wall a-a of the archegonium (Figs. [190], [199] D, E), and developes into the sporogonium, which remains attached to the mother-plant, often nourished by it, as if the two were one organism. The lower extremity of the sporogonium, the foot (Figs. [190] f; [199] D), very often forces its way deep down into the tissue of the mother-plant, but without an actual union taking place. The central portion of the sporogonium becomes a shorter or longer stalk (seta), while the sporangium itself is developed at the summit. At a later stage, during the formation of the spores, the sporangium very often assumes the form of a capsule, and dehisces in several ways characteristic of the various genera (Figs. [192], [193], [194], [195], [200]). The basal portion of the archegonium grows for a longer or shorter period, forming a sheath, the calyptra, in which the capsule is developed, but eventually it ceases to enlarge, and is then ruptured in different ways, but quite characteristically, in each group. Anatomically, the asexual generation is often more highly differentiated than the sexual; thus, for instance, stomata are present on the sporangia of the true Mosses, but are absent in the sexual generation.
As the capsule developes, an external layer of cells—the amphithecium—and an internal mass—the endothecium—are differentiated. As a rule the former becomes the wall of the capsule while the latter gives rise to the spores. In this Division, as in the Pteridophyta, the name archesporium (Fig. [190] t) is given to the group of cells inside the sporangium which gives rise to the mother-cells of the spores. The archesporium is in general a unicellular layer; in Sphagnum and Anthoceros it is derived from the most internal layer of the amphithecium, but with these exceptions it arises from the endothecium, usually from its most external layer. In the true Mosses and in Riccia only spore-mother-cells are produced from the archesporium, but in the majority of the Liverworts some of these cells are sterile and become elaters (cells with spirally thickened walls, Figs. [196], [189]), or serve as “nurse-cells” for the spore-mother-cells, which gradually absorb the nutriment which has been accumulated in them. In Anthoceros, and almost all the Leafy-Mosses, a certain mass of cells in the centre of the sporangium (derived from the endothecium) does not take part in the formation of the archesporium, but forms the so called “column” or “columella” (Figs. [190], [191]).
The spores arise in tetrads, i.e. four in each mother-cell, and are arranged at the corners of a tetrahedron, each tetrahedron assuming the form of a sphere or a triangular pyramid. The mature spore is a nucleated mass of protoplasm, with starch or oil as reserve material. The wall is divided into two layers: the external coat (exospore) which is cuticularized and in most cases coloured (brown, yellowish), and the internal coat (endospore), which is colourless and not cuticularized. On germination the exospore is thrown off, the endospore protrudes, and cell-division commences and continues with the growth of the protonema (Fig. [186], B-D).
Fig. 192.—Andreæa petrophila. A ripe sporogonium: a an archegonium which has been raised with the pseudopodium; p the foot; b the neck; d-e the dark-coloured portion of the sporangium, whose outer cell-walls are considerably thickened; c-c the thin-walled portions where the dehiscence occurs; o the lower extremity of the spore-sac; f calyptra; g the apex of the sporangium. (Mag. 25 times.)
Fig. 193.—Andreæa petrophila. An empty capsule; the calyptra has fallen off. (Mag. 25 times.)
The morphological explanation which Celakovsky has given of the sporogonium, and which is not at all improbable, is, that it is homologous with an embryo consisting of a very small stem-portion and a terminal spore-producing leaf. This will be further explained in the introduction to the Flowering-plants (p. [236]).