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.) Family 3. =Cleistocarpeæ.= The fruit does not dehisce in the regular way, but the spores are liberated by decay. They are small Mosses which remain in connection with their protonema until the sporangium is mature. The archegonium remains sessile at the base of the short capsule-stalk, and is not raised into the air (compare Hepaticæ).--_Phascum, Ephemerum, Archidium, Pleuridium._ Family 4. =Stegocarpeæ.= To this belong the majority of the Mosses, about 3,000 species.