278 THE BRYOPHYTES 



spores have heavy walls and can survive the winter, frequently 

 protected by the spore case. It will be remembered that in the 

 algae the sexually formed spore is generally the protected resting 

 spore. 



285. Alternation of generations. There are thus two phases 

 in the life history of a liverwort or moss. First, there is the 

 plant which bears the sexual organs, and this is called the 

 gametophyte (meaning a gamete-bearing plant) ; second, there is 

 the structure which arises from the fertilized egg and ends its 

 history by developing asexual spores, and for this reason it is 

 called a sporophyte (meaning a spore-bearing plant). The game- 

 tophyte is developed from the spore, and the sporophyte from 

 the fertilized egg. So there is a regular alternation of these two 

 phases in the life history, the gametophyte producing sexual 

 cells, or gametes, and the sporophyte producing asexual spores. 

 The two phases are regarded as separate generations because 

 each has its origin from a distinct kind of reproductive cell 

 (egg or spore). The gametophyte is of course a sexual genera- 

 tion and the sporophyte an asexual one. Their following one 

 after the other makes an alternation of generations, a phrase 

 which from now on will be frequently used, because it signifies 

 the most remarkable feature in the evolution of all plants above 

 the thallophytes. The simple sporophytes of ancient bryophytes 

 gave rise to the fern plants and through them to the large and 

 complicated seed plants. 



A life history which consists of an alternation of sporophyte 

 and gametophyte, as in the liverworts and mosses, may be 

 expressed by the formula 



Gametophyte <^ ' ^> Sporophyte asexual spore 



Gametophyte, etc. 

 This in an abbreviated form becomes 



