358 BOTANY OF THE LIVING PLANT 



of the lamellae are actually the outer surface of the leaf thrown into deep 

 folds, and the leaf itself is part of the gametophyte, not oi the sporophyte. 



An example in the Moss -Plant of extreme simplicity is seen in Buxbauwiid, 

 which habitually grows on humus soil, or rotting tree-stems. Its male plant 

 consists of only a single hollowed leaf, surrounding an antheridium. The 

 female consists of a few leaves, and neither are green. There is an extensive 

 green protonema, but the rhizoids show a hypha-like* habit, and establish 

 very close relations with the humous substratum. The sporogonium itself is 



FIG. 300. 



Half of a transverse section of a leaf of Polytrichum commune, showing the longi- 

 tudinal plates cut in section. 



relatively large in Buxbaumia. The evidence of saprophytism is strong, and 

 it seems probable that many Mosses share that mode of irregular nutrition 

 in varying degree. 



Among the many special adaptations seen in the gametophyte of Mosses 

 one of the most peculiar is that of the Bog-Mosses (Sphagnum) : it is shared 

 in some degree by the quite distinct genus, Leucobryum. The tissues of stem 

 and leaf include not only living cells with active protoplasts, but also dead 

 cells of larger size, with their walls propped out by annular or spiral fibrous 

 thickenings, and opening by round pores to the outside. They form a capillary 

 system by which water is retained as in a sponge. It is this structure which 

 gives Sphagnum its value for surgical dressings. These Mosses occupy large areas 

 under cold wet climates, and their dead bodies are the chief constituent ol peat. 



The permanent establishment of new Moss-Colonies is largely due 

 to the profusion of their methods of vegetative propagation. Pro- 

 tonema is a regular preliminary to the formation of Moss-Plants. A 

 filament may arise from any undamaged cell, either of the plant itself 

 or of the protonema. If a sod on which Mosses are growing is 

 inverted and kept moist, protonema and ultimately a new crop of 

 Moss-Plants will arise from the rhizoids already there. If leaves or 

 stems be chopped up, any undamaged cell may grow out under favour- 

 able circumstances into protonema, giving rise to a new crop. But 



