FERN PLANTS 259 



another kind of spore called an oospore, which is capable 

 of germinating. It reproduces, however, not the simple 

 thalluslike gametophyte from which it sprang, but the 

 beautiful fern plant, or sporophyte, with its vascular system 

 and complete outfit of vegetative organs — root, stem, 

 and leaves. 



368. Alternation of Generations. — We all know the 

 meaning of the word generation as applied to the direct 

 descendants of one organism from another, whether animal 

 or plant. When two successive generations produce respec- 

 tively ordinary spores and oospores, and these different 

 kinds of spores give rise to organisms unlike in structure 

 or habits of life, there is said to be an alternation of 

 generations. The generation which bears the simple spores 

 (sporophyte) is said to be asexual ; the one which produces 

 the gametes and oospores is sexual ; that is, it requires the 

 union of two separate bodies to produce a fertilized germ, 

 or oospore. Each generation, therefore, it will be observed, 

 gives rise to its opposite, the asexual sporophyte producing 

 the sexual gametophyte, or prothallium, and this in turn, 

 through its gametes and oospores reproducing the asexual 

 sporophyte. The alternation in ferns may, in general, 

 be expressed to the eye by a series of diagrams like those 

 given below. The words in each line are synonyms of 

 those immediately above or below them in the other lines, 

 except it must be observed that, strictly speaking, it is 

 not the antheridia and archegonia, but the spores or 

 gametes contained in them that by their union produce 

 the oospore. 



Fern plant— >- Spores —>- Prothallium —>-^ , .,. ^—>-'Oospore—^ Fern plant. 



Sporophyte — >- .Spores — >- Gametophyte — ^ <^ y —^ Oospore — >- Sporophyte. 



c . / Gamete \ ^.. , , 



Asexual gen. -^- Spores — >- Sexual gen. — ^( ^—>- Oospores— >- Asexual gen. 



369. Advantages of Alternation. — This roundabout 

 mode of reproduction would hardly have been developed 



