The calyptra is either simply mitre-shaped, or mítriform ([Pl. III.] fig. 24), or it is half-cleft, or dimid´iate (figs. 14, 36). When the capsule is ripe, the upper part usually separates at a circular horizontal line (fig. 8) as a kind of lid, which is called the oper´culum (operculum, a lid), and thus the spores are enabled to escape. The rim of the capsule, from which the operculum has separated, forms its mouth, and this often exhibits a fringe of teeth (figs. 15, 18, 31), arranged in one or more rows; sometimes the teeth are replaced by a membrane, or, again, both teeth and a membrane may be present. This mouth-fringe is the per´istome (περἰ, around, στὁμα, mouth). In many mosses, an elastic row or ring of cells is situated between the mouth of the capsule and its operculum, called the annulus (figs. 18 & 39); this, when the capsule is ripe, aids in throwing off the operculum.
It is important to become acquainted with the structure and arrangement of these parts, as they form characters by which the families and genera of mosses are distinguished.
The capsules of the mosses form very beautiful microscopic objects, especially those furnished with a toothed peristome.
Most of the mosses produce their fructification in the winter and spring.
The class of mosses is divided into two Orders, according to whether the fruit-stalk is terminal, i. e. arises from the end of the stem or its branches, or whether it is lateral, arising from the side of the stem. Those with the fruit-stalk terminal, or the end fruited ([Pl. III.] fig. 22), form the Ac´rocarpi (ἄκρα, summit, καρπὀς, fruit); while those with the fruit-stalks lateral, or the side-fruited mosses (fig. 43), constitute the Pleu´rocarpi (πλευρἀ, side). The new shoots or young branches of the stems of mosses are termed innovations.