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.
The capsule opens as in Sphagnum by means of a lid (operculum), which is often prolonged into a beak. Round the mouth of the opened capsule, a number of peculiar yellow or red teeth are to be found. These constitute the peristome; their number is four, or a multiple of four (8, 16, 32 or 64). The form and thickenings of these teeth are widely different, and on this account are used by Systematists for the purposes of classification. In some Mosses (Fig. [200] C, D) there is a double row of teeth. Except in Tetraphis they are not formed from entire cells, but from the strongly thickened portions of the wall of certain layers of cells belonging to the lid, and persist when this falls off. They are strongly hygroscopic, and assist greatly in the ejection of the lid, in which operation they are considerably aided by a ring of elastic cells with thickened walls, situated in the wall of the lid near the base of the teeth. This ring is known as the annulus. The archegonium is raised into the air like a hood, the calyptra, which either covers the sporangium on all sides (having the shape of a bell), or is split on one side (Fig. [200] B, h).
Among peculiar forms may be mentioned: Splachnum, which is especially remarkable for the collar-like expansion at the base of the capsule. Fissidens deviates in having a flat stem and leaves arranged in two rows. The leaves are boat-shaped and half embrace the stem.—Schistostega has two kinds of stems. The barren ones resemble Fern-leaves; they have two rows of leaves, which are attached together vertically, are decurrent and coalesce at their bases. The fertile ones have an ordinary appearance.—Tetraphis: the peristome is composed of four teeth, which are formed from entire cells. T. pellucida has peculiar gemmæ.
The family is divided into two groups: the Musci acrocarpi, the growth of whose main axis is limited and terminated by the formation of the sexual organs; and the Musci pleurocarpi, whose sporogonia are situated on special lateral shoots, while the growth of the main axis is unlimited.
Fig. 200.—A Hypnum populeum. B and C Sporangia, with hood (h), and operculum (l’), and without these (C), showing the peristome (p). D The mouth of the capsule of Fontinalis antipyretica.