If we have followed carefully the development of marchantia, the study of one of the mosses will be comparatively easy. The mosses are more familiar plants than the liverworts. They grow on trees, stones, and on the soil in both wet and dry places. One of the common larger mosses, known as _Polytrichum commune_, may serve as an example, Fig. 295. This plant grows on rather dry knolls, mostly in the borders of open woods, where it forms large beds. In dry weather these beds have a reddish brown appearance, but when moist they form beautiful green cushions. This colour is due, in the first instance, to the colour of the old stems and leaves, and, in the second instance, to the peculiar action of the green living leaves under the influence of changing moisture-conditions. The inner or upper surface of the leaf is covered with thin, longitudinal ridges of delicate cells which contain chlorophyll. These cells are shown in cross-section in Fig. 296, as dots or granules. All the other tissue of the leaf consists of thick-walled, corky cells which do not allow moisture to penetrate. When the air is moist the green leaves spread out, exposing the chlorophyll cells to the air, but in dry weather the margins of the leaves roll inward, and the leaves fold closely against the stem, thus protecting the delicate assimilating tissue. [Illustration: FIG. 296.—SECTION OF LEAF OF POLYTRICHUM COMMUNE.] The _antheridia_ and _archegonia_ of polytrichum are borne in groups at the ends of the branches on different plants (many mosses bear both organs on the same branch). They are surrounded by involucres of characteristic leaves termed _perichætia_ or _perichætal leaves_. Multicellular hairs known as _paraphyses_ are scattered among the archegonia and antheridia. The involucres with the organs borne within them are called _receptacles_, or, less appropriately, “moss flowers.” As in marchantia, the organs are very minute and must be highly magnified to be studied. [Illustration: FIG. 297.—SECTION THROUGH A RECEPTACLE OF POLYTRICHUM COMMUNE, showing paraphyses and antheridia. ] The antheridia are borne in broad cup-like receptacles on the antheridial plants (Fig. 297). They are much like the antheridia of marchantia, but they stand free among the paraphyses and are not sunk in cavities. At maturity they burst and allow the sperm-cells or _spermatozoids_ to escape. In polytrichum, when the receptacles have fulfilled their function, the stem continues to grow from the centre of the cup (_m_, Fig. 295). The archegonia are borne in other receptacles on different plants. They are like the archegonia of marchantia except that they stand erect on the end of the branch.