The ventral portion of the archegonium is very early ruptured at its base by the growing sporogonium, upon which it remains, and it is thus raised into the air, forming a “hood,” the calyptra (Figs. [192]; [200] B). In the Sphagnaceæ the hood is not present; in this order, as in the Liverworts, the archegonium remains at the base of the sporogonium. The sporangium opens by circumsessile dehiscence, the upper portion (operculum) being separated along a specially constructed ring of cells, and falls off like a “lid” (Fig. [200]). Only in a few forms (families 2 and 3) does any variation of this take place. Elaters are never found, but (with the exception of Archidium) there is always present in the sporangium a central mass of cells, the columella, which take no part in the formation of the spores. The columella, in some, does not reach quite to the operculum and in these cases the spore-sac is bell-shaped and covers the columella (Andreæa, Fig. [190]; Sphagnum, Fig. [199] D); but in the majority of Mosses the columella extends to the lid, so that the space containing the spores becomes a hollow cylinder.
The sporangium is generally raised on a long stalk; in the great majority this stalk is formed from the lower half of the oospore and belongs to the asexual generation—it is then known as the seta. In Andreæa and Sphagnum the seta is very short, and the sporangia are raised upon a long stalk (pseudopodium) developed from the summit of the sexual generation (Figs. [190], [192]). In the latter figure an archegonium (a) is seen attached to the pseudopodium, having been carried up with this during the course of its development. The summit of the pseudopodium is enlarged to embrace the foot of the sporogonium (Figs. [192], [199] D).
A. The sporangium is supported on a pseudopodium; the columella does not extend to the operculum.
Fig. 199.—Sphagnum acutifolium.—A The upper portion of a plant: a branches with antheridia; ch branches with terminal archegonia and perichætia; b the upper stemleaves. B A male branch whose leaves are partly taken off in order to show the antheridia. C Group of three archegonia: the central one (a) is formed from the apical cell. D Sporogonium in longitudinal section: the broad foot (sg’) is sunk in the vaginula, v; c calyptra; ar neck of the archegonium; ps pseudopodium. E ripe sporangium with operculum, and the remains of the archegonium situated on the pseudopodium which is still surrounded by the perichætium; to the left is a barren branch. F Portion of a foliage-leaf seen from above: l perforations; b chlorophyll-containing cells; s spiral thickenings.
Family 1. Sphagneæ (Bog-Mosses).
The protonema has been already described. The stem is regularly branched owing to the fact that a branch, or collection of branches, arises at every fourth leaf. These branches are closely covered with leaves, some are erect, while others hang down and surround the stem. No rhizoids are developed. These Mosses are of a whitish-green colour, and when water is present are always saturated with it like a sponge, the reason for this being found in the construction of the stem and leaves. The stems are covered by an external layer of large clear cells, without chlorophyll, but with annular or spiral thickenings on the walls, which are also perforated by large holes. By means of capillary attraction, water is thus raised to the summit of the stem. Similarly constructed cells are also found in the leaves, but they are surrounded by a net of very narrow, chlorophyll-containing cells (Fig. [199] F), whose colour is thus to a great extent lost amongst those which are colourless. This anatomical structure is an essential condition for the formation of peat. The Bog-Mosses grow by preference on moors, which they cover with a thick carpet saturated with water. The lower extremities of the plants perish very rapidly, and gradually become converted into peat, and the branches thus separated from each other become independent plants. The sporangia (Fig. [199] D, E) are spherical, but with a very short stalk. They open by a lid, but have no annulus. The archegonium (Fig. [199] C) persists at the base of the sporogonium as in the Liverworts. Only one genus, Sphagnum.
Family 2. Schizocarpeæ.
The Mosses which constitute this family are of a brownish-black colour and are found living on rocks. The sporangium resembles that of the Liverworts inasmuch as it opens by four valves, but these continue attached to each other at the apex as well as at the base (Fig. [193]).—There is only one genus: Andreæa.
B. The stalk is formed from the lower portion of the sporogonium. The columella is continued to the summit of the sporangium and united with it (Archidium has no columella.)