BEYOPHYTES 



97 



duces only sexual spores (oospores), and therefore pro- 

 duces sex organs and gametes. It is known, therefore, 

 as the gametophyte — that is, "the gamete plant.'' 



The generation which consists of tho "spore fruit" — ■ 

 that is, leafless stalk and spore case — produces only asexual 

 spores, and is called the sporophyte — that is, "the spore 

 plant." 



Alternation of generations, therefore, means the alter- 

 nation of a gametophyte and a sporophyte in completing a 

 life history. Instead of having the same body produce both 

 asexual and sexual spores, as in most of the Algae, the two 

 kinds of spores are separated upon different structures, 

 known as "generations." It is evident that the gameto- 

 phyte is the sexual generation, and the sporophyte the 

 asexual one ; and it should be kept clearly in mind that 

 the asexual spore always produces the gametophyte, and 

 the sexual spore the sporophyte. In other words, each 

 spore produces not its own generation, but the other one. 



The relation between the two alternating generations 

 may be indicated clearly by the following formula, in 

 which G and S are used for gametophyte and sporophyte 

 respectively : 



Gzz8 > 0— S— 0— G=§ > 0— S— 0— G, etc. 



The formula indicates that the gametophyte produces 

 two gametes (sperm and egg), which fuse to form an oospore, 

 which produces the sporophyte, which produces an asexual 

 spore, which produces a gametophyte, etc. 



That alternation of generations is of great advantage is 

 evidenced by the fact that it appears in all higher plants. 

 It must not be supposed that it appears first in the Bryo- 

 phytes, for its beginnings may be seen among the Thallo- 

 phytes. The Bryophytes, however, first display it fully 

 organized and without exception. Just what this alterna- 

 tion does for plants may not be fully known, but one 

 advantage seems prominent. By means of it many gameto- 

 phytes may result from a single oospore ; in other words. 



