100 FOUNDATIONS OF BIOLOGY 



complications. Beproduction, divested of its specialized 

 features, is merely growth expressed in cell divisions. This 

 primary potentiality of all cells may exist side by side 

 with the development of cells specialized for asexual repro- 

 duction (spores) and sexual reproduction (gametes). In 

 either case the products become separated from the parent 

 body and develop new bodies. Furthermore, spores which 

 at first are developed from any of the vegetative cells of the 

 plant body, later arise in asexual reproductive organs 

 (sporangia), while gametes are produced in sexual reproduc- 

 tive organs (gametangia) . With the morphological differen- 

 tiation of gametes into sperm and eggs, a further specializa- 

 tion of the gamete-forming organs results in male and female 

 reproductive organs (antheridia and archegonia). When 

 sporangia and gametangia are borne by separate individuals, 

 asexual plants (SPOROPHYTES) and sexual plants (GAMETO- 

 PHYTES) result. Finally, the sperm and eggs may be borne 

 on separate gametophytes, in which case male and female 

 gametophytes result. 



E. ALTERNATION OF GENERATIONS 



From the standpoint of the evolution of the higher plants 

 the most significant fact stated above is that sporangia and 

 gametangia may be borne by separate individuals, for this 

 clearly involves an asexual, spore-bearing generation, and a 

 sexual, gamete-bearing generation. We shall outline this 

 alternation of generations in the life history of a typical Moss 

 and Fern as an introduction to the problem of reproduction 

 in the higher Flowering Plants. 



1. The Moss 



The common Mosses of woods, hillsides, and fields are a 

 relatively inconspicuous but nevertheless an important part 



