Order 20. =Hypnaceæ.= The leaves are smooth with square, often bladder-like, cells at the edge. _Hylocomium_ (_H. splendens_, _H. triquetrum_); _Hypnum_; _Brachythecium_; _Plagiothecium_.--_Eurhynchium._--_Homalothecium_, _Isothecium_, _Orthothiecium_, _Homalia_.--_Climacium_, _Lescuræa_, _Leucodon_. The Mosses occur all over the globe. Many are found in great numbers, and growing thickly massed together, they form an important feature in landscapes (for example _Sphagnum_ and _Polytrichum_ in the Arctic Tundra). In the Northern and Arctic regions the Mosses are very plentiful, and often form a considerable part of the vegetation, while in the Tropics they are insignificant. Species of _Hypnum_ and _Polytrichum_, like _Sphagnum_, play an important part in the formation of peat. DIVISION III. PTERIDOPHYTA (VASCULAR CRYPTOGAMS). The alternation of generations is as distinct in this Division as in the Mosses, but the sexual generation consists of only a small thallus, the prothallium, which bears directly the sexual organs, _antheridia_ and _archegonia_; and the asexual generation, which arises from the fertilisation of the oosphere, is no longer a single short-lived sporangium, but a highly developed, generally perennial, plant provided with stem, leaves and _true roots_ (Ferns, Horsetails, etc.), the sporangia being borne on the leaves. In this latter generation the tissues are differentiated into epidermis, ground tissue and vascular tissue; in the last named the bundles are closed, and in the majority of cases concentric. The =sexual generation=, =gametophyte=, or =prothallium=, is _always a thallus_, although not always green and leaf-like (Figs. 205, 215, 222, 229, 235, etc.) It is very small, even in cases where it attains the greatest development, and consists only of parenchymatous cells. The prothallium is nourished by hair-like roots (rhizoids) and has only a transitory existence, dying soon after the fertilisation of its oosphere. The ANTHERIDIA exhibit great variations in structure which, however, must be considered as modifications of the fundamental type which is found in the Mosses. These modifications will be mentioned under the various families. The _spermatozoids_ are always spirally-coiled, self-motile, protoplasmic bodies, with most frequently a large number of fine cilia on the anterior end (Figs. 206, 223, 234). They are formed principally from the nucleus of the mother-cell, and portions of the cytoplasm often remain for a time attached to their posterior end.