320 



BOTANY: PRINCIPLES AND PROBLEMS 



foot, its wall is thick, and much of the central tissue is sterile, 

 the spores occurringjn a rather small, dome-shaped mass in the 

 upper portion. There is no true seta but 

 the sporophyte is carried upward on a stalk 

 formed by the gametophyte. 



2. Bryales or True Mosses. — These are 

 the common mosses, numbering over 12,000 

 species and widely distributed over the 

 globe. Many of them are particularly well 

 suited to live in cold or dry situations and 

 they often form the vanguard of an ad- 

 vancing vegetation. 



The spore does not germinate directly 

 into what we know as a moss plant, but 

 produces instead a mass of green filaments, 

 the protonema (Fig. 194). From this arise 

 erect branches which grow into the leafy 

 shoots with which we are familiar (Figs. 

 (i and 195). The stem is usually but a few 

 centimeters in height and shows little com- 

 plexity, although there may often be dis- 

 tinguished a firm central mass of tissue — 

 presumably the region of conduction — and 

 a softer zone outside. Nothing approaching 

 the highly differentiated stem structure of ferns and seed plants 



Fig. WS.^ Sphagnum. 

 Leafy shoot of the game- 

 tophyte, with three cap- 

 sules at the top. 



Fig. 194. — Moss protonema, the deHcate, thread-Hke structure which grows 

 from the germinated moss spore. Along this protonema several young moss 

 plants are arising, the one at the left well started, the other three mere buds. 



is present, however. The leaves are typically small and narrow, 

 and but one layer of cells in thickness. They may often become 



