492 BOTANY PART n 



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 and are 

 generally provided with a median 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 water. These cells are devoid 

 of protoplasm, and are similar to those in the periphery of the stem, but larger 

 and more elongated ; their walls, which are perforated, are strengthened by 

 transverse thickening bands. Between them are other elongated, reticulately 

 united cells containing cbloroplasts. A similar differentiation of the leaf -cells 

 occurs in a few other Mosses (e.g. Leucdbryum glaucum}. 



A more complicated structure of the leaves resulting from their adaptation to 

 the absorption of water and protection against drying is exhibited by Polytrichum 

 commune. In this Moss the leaves develop on their upper surface numerous, 

 crowded, vertical lamellae, one cell thick ; these contain chlorophyll and serve 

 as an assimilatory tissue, while the spaces between the lamellae serve as reservoirs 

 for the storage of water. In a dry atmosphere the leaves fold together, and thus 

 protect the delicate lamellae from excessive transpiration ( 107 ). Many Mosses can 

 endure desiccation without injury. 



The RHIZOIDS (Figs. 454, 456), each of which consists of a branched filament of 

 cells without chlorophyll, spring from the base of the stem. In structure they 

 resemble the protonema, into which they sometimes become converted, and then 

 can give rise to new moss plants. 



The SEXUAL ORGANS are always borne in groups at the apices 

 either of the main axes or of small, lateral branches, surrounded by the 

 upper leaves of the latter which frequently have a distinctive structure, 

 and are known as the PERICHAETIUM (Fig. 456). Between the sexual 

 organs there are usually present a number of multicellular hairs or 

 paraphyses. The moss plants may be monoecious, in which case both 

 kinds of sexual organs are borne on the same plant either in the same 

 or different receptacles ; or dioecious, and then the antheridia and arche- 

 gonia arise on different plants. The archegonia and antheridia of 

 Mosses differ in their development from those of other Archegoniatae 

 by being formed by the segmentation of a two- or three-sided apical cell. 



The SPOROGONIUM of the Mosses ( 108 ) develops a capsule with an 

 axial COLUMELLA consisting of sterile tissue (Fig. 458). The spore-sac 

 surrounds the columella, which conducts and accumulates food material 

 and water for the developing spores. Elaters are never formed. In the 

 young sporogonium outside the spore-sac a well-developed assimilating 

 tissue is present; this is bounded by water-storage tissue and an 

 epidermis. In most Mosses stomata are found on the lower part 

 of the capsule. The ripe capsule exhibits a variety of peculiar 

 structures to facilitate the opening and the distribution of the spores. 

 The stalk or seta raises the capsules so that the spores are readily 

 dispersed by wind. Distinctive variations in the mode of develop- 

 ment and structure of the capsules are exhibited by the three orders 

 of the Musci : Sphagnales, Andreaeales, and Bryales. 



