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.
The ARCHEGONIA are more uniform throughout the entire Division, and more closely resemble those of the Mosses. They are, as in the previous Division, principally flask-shaped; but the central portion, which encloses the oosphere, is always embedded in the tissue of the prothallium, so that the neck, which is formed of 4 rows of cells, projects above the surface (Figs. [201] 3, [222] h). The development of the archegonium in a Fern is seen in the accompanying figure (Fig. [201]). The archegonium is developed from a surface cell, which divides into three cells by two walls in a direction parallel to the surface of the prothallium (Fig. [201]). The most internal cell becomes the ventral portion of the archegonium. The external one (b) divides perpendicularly to the surface of the prothallium into four cells, which again divide parallel to the surface and form the neck (b, in 2 and 3). The intermediate cell projects upwards into the neck and divides into two, the lower one, after the separation of the ventral canal-cell, becoming the oosphere, and the upper one the neck-canal-cell (c, in 2 and 3).
Fig. 201.—Pteris serrulata. Development of archegonia.]