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_. We will now examine a few common mosses more in detail, beginning with the ACROCARPI. _Sphag´num acutifólium_ (Pl. III. fig. 4) is found in pools or bogs, growing at the margins so as to be partially immersed. In this moss, the upper branches are grouped into a head. The leaves are crowded, and overlapping or im´bricate (_imbrex_, a tile) on the elongate stems; they are egg-shaped (ovate) on the main stems (fig. 1), and narrower or ovate-lanceolate on the branches; they are nerveless, and finely toothed at the apex. The capsule (fig. 5) is roundish-ovate, without a peristome, and the operculum is flattened. The grouped arrangement of the upper branches renders the species of _Sphagnum_ easily recognized. The structure of the leaves is also very peculiar and characteristic (fig. 2). The cells of which they consist are of two kinds, one (fig. 2 _a_) being colourless, elongate, pointed, and containing a spiral fibre; the other consisting of shorter and narrower obtuse cells, containing chlorophyll, and situated between the former. In many of the former kind of cells, little round apertures exist on the under surface, and minute animals may sometimes be found imprisoned in them. Another species of _Sphagnum_, _S. obtusifolium_, is common, and greatly resembles the above, but has shorter and thicker stems, and rounded-ovate, very concave, and obtuse leaves. _Gymnos´tomum truncat´ulum_ (Pl. III. fig. 6) is a common little moss, found on banks and in fields and gardens.