The species of the acrocarpous tribe Phascei are exceedingly numerous, and contain the simplest of all mosses; they grow on newly turned-up soil, and are chiefly annual. Their leaves generally have nerves, and are bordered by large cells. The urns, which are either sessile, or upon a short stalk, have not a trace of peristome, and sometimes have a columella, sometimes not. The spores are large compared with those of other mosses. The tribe Dicranei contains numerous species, some of which are the commonest of mosses in Europe. They are easily known by their single peristome of sixteen teeth divided half-way down. The leaves are extremely crisp and convolute, and the hood is spoon-shaped. The Leucobryum is remarkable for pallid leaves; it has three layers of cells, a narrow layer of green cells embedded in the centre of the leaf with a broader layer of colourless cells on each side, whose cell walls are perforated with large round openings, as in fig. 50 _b_. The mosses of this order live on sandstone rocks, shady banks, and trunks of trees. Fig. 50 shows the microscopic structure of the leaves of various mosses. In the tribe Grimmiei we have frequently a sessile urn, with a single peristome, and a mitre-shaped hood. The leaves, which are dark green, have minute hexagonal perforated cells on their upper surface, and a white nerve projecting from their extremity. In the tribe Polytrichei the mouth of the urn is mostly closed by a flat membrane and a hood rough with silky hairs. The leaves are sheathing at their base and spreading at their tips; except in a few cases they are rigid; and the nerve often exhibits lamelliform folds. The Polytrichum dendroides contains scalariform ducts, and starch granules. The Bryei are of variable size, but a number of European species are among the finest of mosses, on account of their large leaves and beautiful double-toothed peristome, the great distinction of the group. The leaves are margined, toothed, and composed of a loose reticulation of large rhomboidal cells; in the genus Timmia they clasp the stem at the base and spread widely at the tip. There are thirty-three British species of the genus Bryum, many of which with their abundant urns are extremely ornamental. [Illustration: Fig. 50. Microscopic structure of leaves of mosses:—_a_, Octoblepharum albidum; _b_, Leucobryum glaucum; _c_, Sphagnum latifolium; _d_, Hypopterygium Smithianum; _e_, Eucamptodon perichætialis; _f_, Andræa subulata; _g_, Campylopus lamellinervis.]