MOSSES 181 
and also produces the sex-organs. The life-history, with 
its alternating generations, may be indicated as follows: 
filament and)—o stalked —o 
G (eafy branch) # o—5 adore pane = > jp G, 
ete. 
104. The great groups of Mosses.—There are two great 
groups of mosses, known as the bog mosses and the truc 
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 
