MOSSES 181 



and also produces the sex-organs. The life-history, with 

 its alternating generations, may be indicated as follows: 



etc. 



104. The great groups of Mosses. There are two great 

 groups of mosses, known as the bog mosses and the true 

 mosses. The bog mosses are large and pallid mosses found 

 abundantly in bogs and marshy ground, and are the most 

 conspicuous peat formers. They differ from the true mosses 

 in structure in many ways that need not be mentioned, 

 but one contrasting character deserves attention. When 

 the spore of a bog moss germinates, it does not produce a 

 branching green filament, but a flat compact thallus body 

 like that of the liverworts. On this thallus body the 

 erect leafy branches arise, just as they do from the fila- 

 mentous body in true mosses. This is interesting, because 

 in the bog mosses the thallus body of the liverworts is 

 continued, and also because it indicates that the prostrate 

 filamentous body of the true mosses is probably a modified 

 thallus body. 



The true mosses are much more numerous than the bog 

 mosses, and live in a far greater variety of situations. 

 Some of them are also peat formers, but most of them have 

 become established in much drier situations. 



105. The erect leafy axis. The lowest green plants live 

 in the water or in very moist places, but the liverworts 

 begin to occupy the land. In this new position they are 

 better exposed to light, which is an advantage in food 

 manufacture; but they are in danger of being dried out by 

 the air. In consequence of these dangers, various protect- 

 ive structures have been developed, one of the first being a 

 compact body with an epidermis. An exposure of more 

 green tissue to the light is secured by the leafy liverworts 

 in their development of leaves, but their bodies are prostrate 



