390 BOTANY 



by an involucre, a sheath-like outgrowth of the thallus or stem {e.g. Blasia pusilla, 

 Fig. 11). 



(b) Acrogynom Jungermanniaceae. — This group includes only foliose forms {e.g. 

 Plagiochila asplenioides). The sporogonia arise apically from the extremities 

 of the stem or its branches, and are surrounded by a perianth formed of special, 

 characteristically-shaped leaves (Fig. 12). The majority of the Jungermanniaceae 

 resemble the true Mosses ; they are small and grow on the ground or on tree-trunks, 

 and in the tropics also on the leaves of forest plants. 



Class II 

 Musci (Mosses) 



The profusely -branched protonema of the Mosses appears to 

 the naked eye as a felted growth of fine, green filaments (Fig. 312). 

 Buds are developed on the protonema, which grow by means of a 

 three-sided apical cell, and give rise to Moss-plants, which always 

 exhibit segmentation into stem and leaves. The leafy Mosses may 

 be readily distinguished from the foliose Jungermanniaceae by the 

 spiral arrangement of their small leaves, which are rarely arranged in 

 two rows. In Mosses which have prostrate stems, the leaves, although 

 arranged spirally, frequently assume a somewhat outspread position, 

 and all face one way, so that in such cases a distinction between an 

 upper and a lower side is manifested, but in a manner different from 

 that of the Liverworts. 



The stem of the Moss-plant is formed of cells which become 

 gradually smaller and thicker -walled towards the periphery. In 

 the stems of many genera (e.g. Mnium, Fig. 159, p. 147) there is 

 found a central, axial strand consisting either of elongated, conduct- 

 ing cells with narrow lumina and devoid of protoplasm, or of such 

 empty cells together with others filled with protoplasmic contents. 

 These strands, which are not always present, may be regarded as 

 incipient vascular bundles. They do not occur, for instance, in the 

 genus Sphagnum, which grows in swampy places. The stems of this 

 Moss show a peculiar development of the outer cortical layers (Fig. 

 320, C). The cells in these layers are devoid of protoplasm, and are 

 in communication with each other and the atmosphere by means of 

 large, open pores ; to secure rigidity, they are also provided with 

 spirally thickened walls. They have a remarkable power of capillary 

 absorption, and serve as reservoirs for storing and conducting water. 



The leaves of the true Mosses have, as a rule, a very simple 

 structure. They consist usually of a single layer of polygonal cells 

 containing ' chloroplasts (Fig. 55, p. 56; Fig. 72, p. 68), and are 

 generally provided with a conducting bundle of elongated cells. The 

 leaves of the Bog Mosses (Sphagnaceae) have no bundles, and instead 

 are supplied with capillary cells for the absorption and storage of 



