[Chap. XLVIII MOSSES AND LIVERWORTS 649 



Chromosomes and the Hfe cycle of mosses. Since reduction division 

 occurs during the formation of spores in the sporophyte, these spores and 

 all cells of the gametophvte have the n, or monoploid, number of chromo- 

 somes; while the cells of the sporoph\ te, which develops from the fer- 

 tilized egg, have the 2m, or diploid number. Moreover, all the cells of 

 the gametoph\ te, including the protonema with all of its leafv branches 

 and all of their sperms and eggs, have the same chromosome comple- 

 ment. Consequentlv if self-fertilization occurs, each of the fertilized eggs 

 and resulting sporophytes will have two sets of like chromosomes. They 

 will be completelv homozvgous. If reduction division is regular, all the 

 spores fomied in these sporophytes will have the same chromosome com- 

 plement, and so will all of the subsequent gametophytes that develop 

 from them. Similar chromosome phenomena mav occur in bisexual liver- 

 worts and ferns also. 



Classification of mosses. The foregoing description of a moss life cycle 

 refers especiallv to the group sometimes called the "true mosses" 

 (Brijales). Two other groups or orders of mosses are generally recog- 

 nized: the peat or bog mosses {Sphagnales) and the rock or granite 

 mosses (Andreaeales). These differ from the Bnjales in various struc- 

 tural details of both gametophvtes and sporophytes. Descriptions of these 

 orders mav be readilv found in books on the mosses, some of which are 

 listed at the end of this chapter. 



The peat mosses, Sphagnales, differ from other mosses in several char- 

 acteristics. The protonema is a flat thallus similar in appearance to a 

 small liverwort or the prothallus of a fern. It begins as a filament, but 

 soon becomes a flat plate one-cell thick and irregularly lobed ( Fig. 299 ) . 

 The visible stalk ( pseudopodium ) upon which the capsule is borne is an 

 outgrowth of the gametophvte and is not the lower part of the spor- 

 ophyte. The sporophyte consists of a globular capsule, a very short and 

 slender stalk, and a foot. The spore-bearing part of the capsule is dome- 

 shaped, overarching a region of sterile tissue. The leaves are a single 

 cell laver in thickness, have no midrib, and contain both living and dead 

 cells. In \ouno; leaves all the cells look alike; but later, differentiation 

 results in slender living cells, which possess chloroplasts and fonn a 

 network enclosing larger, short-lived, non-green cells. 



The peat mosses are of more commercial value than any others. These 

 plants often form floating masses on the surface of lakes and ponds. 

 Continued growth of the plants upward forces lower layers downward, 

 and the dead and decaying parts accumulate, disintegrate but little in 



