192 PLANT MORPHOLOGY 



ity is of an entirely different kind. The gametophyte is a flat thallus 

 without structural differentiation. It grows by means of a cuneate apical 

 cell. The sex organs are endogenous. The antheridium is formed from 

 the inner half of the initial cell and the sequence of early wall formation 

 is distinctive. The archegonium represents a new departure in that the 

 primary neck canal cell is cut off from the outer segment arising from a 

 transverse division of the initial. 



The sporophyte consists of a foot and a cylindrical capsule. It dis- 

 plays a great development of sterile tissue. The presence of green tissue, 

 stomata, and rhizoid-like processes suggests that the sporophyte is becom- 

 ing independent. The breaking up of the sporogenous tissue into smaller 

 units may represent an initial stage in the formation of sporangia. The 

 establishment of a sterile axis by the transfer of sporogenous tissue from 

 the endothecium to the amphithecium may represent the beginning of a 

 conducting system. A meristematic region in the sporophyte results in 

 its continued growth. Dehiscence of the capsule is accomplished by 

 means of two valves. 



2. MUSCI 



The mosses constitute the larger and more highly developed class of 

 bryophytes, numbering about 14,000 species. They are widely distrib- 

 uted and, although abundant in arctic and alpine regions, are represented 

 in nearly all habitats except the ocean. The fossil history of the group is 

 very fragmentary, there being few reliable evidences of its existence 

 earlier than the Tertiary. The gametophyte is leafy and, in contrast to 

 that of the liverworts, is typically radial rather than dorsiventral. The 

 leaves generally have a midrib. The rhizoids are septate and usually 

 branched. The Musci comprise three orders, the Sphagnales, Andreaea- 

 les, and Bryales. 



1. Sphagnales 



The Sphagnales, or bog mosses, are a group of about 350 species, all 

 belonging to the genus Sphagnum. They are relatively large, pale mosses 

 generally living in bogs at high altitudes and high latitudes. Because 

 their accumulated remains form peat, they are often called peat mosses. 



Gametophyte. Upon germination, the spore produces a short filament 

 that, in turn, gives rise to a flat green thallus (Fig. 157 A). This consists 

 of a single layer of cells bearing numerous septate rhizoids. The thallus 

 gives rise to an erect leafy branch and then disappears (Fig. 1575). The 

 erect shoot develops rhizoids below and becomes the mature gametophyte. 

 The rhizoids soon die but the shoot continues to grow from year to year 

 (Fig. 158A). Branching in Sphagnum is very profuse, there being 

 branches of limited growth crowded near the apex of the main stem and 

 others occurring in tufts farther down. The stem grows by means of a 



