LIVERWORTS 171 



and absorbs the necessary nourishment from it. When 

 ripe, the spore-case is ruptured (Fig. 167, B), and the light 

 spores are scattered; and when they germinate they pro- 

 duce new thallus bodies. 



96. Alternation of generations. The life-history of M ar- 

 chantia shows a distinct alternation of generations, and 

 since this is a feature of all the higher plants it is necessary 

 to understand it clearly. The thallus body produces no 

 spores, but produces sperms and eggs; that is, it produces 

 gametes, and hence is called a gametophyte, which means a 

 "gamete plant." The gametes produce an oospore; but 

 the oospore does not produce a new thallus plant, producing 

 instead a spore-case. This structure, called the spore-case, 

 does not produce gametes, but produces spores; and hence 

 it is called a sporophyte, which means a "spore plant." 

 Thus in the life-history of Marchantia and of all higher 

 plants, there is an alternation of gametophyte and sporo- 

 phyte. It is evident that in this alternation of generations 

 the gametophyte is the sexual and the sporophyte is the 

 sexless generation. Therefore, oospores are produced by the 

 gametophyte, and ordinary spores by the sporophyte; but 

 the oospores always produce sporophytes, and the ordinary 

 spores always produce gametophytes. These relations may 

 be indicated clearly by the following formula, in which G 

 and S are used for gametophyte and sporophyte respec- 

 tively : 



S o G, etc. 



The formula indicates that the gametophyte produces 

 two gametes, which fuse to form an oospore, which produces 

 the sporophyte, which produces an ordinary spore, which 

 produces a gametophyte, etc. It will be remembered that a 

 similar alternation of generations was noted in the red 

 Algae ( 75) and in the mildews ( 82) among the Thallo- 



