272 BIOLOGY 



and enable them to swim in the water, which moistens the 

 under surface of the prothallium. In thie moisture they swirn 

 in all directions, and some of them come to the mouths of the 

 archegonia. When this occurs there is an attraction between 

 the egg at the bottom of each archegonium and the sperm which 

 has reached its top; the sperm swims to the egg and fuse:- 

 with it, i. e., fertilizes it. 



After the egg has been fertilized by the sperm, it is endowed, 

 like any other fertilized egg, with the power of growth. It 

 soon begins to divide, grows rapidly, and develops in the course 

 of time into a little plant which, by continued growth, becomes 

 the fern with which we are familiar and like that with which we 

 started the history; Fig. H, f. Thus we see that the common 

 fern grows from a fertilized egg, and that the spore produced 

 by the fern grows, not into a fern at first, but rather into a 

 prothallium. 



The life history of the fern is thus an alternation of two 

 different stages and two different methods of reproduction. 

 There is first the fern proper, which, since it produces only 

 spores, is the asexual stage of the plant, and is called the sporo- 

 phyte (Gr. sporos + phyton = plant). The second stage is the 

 prothallium, which, since it produces eggs and sperms, is the 

 sexual stage. This is called the gametophyte (Gr. gamete -f- 

 phyton) stage, since it produces gametes. Each of the thou- 

 sands of spores of the fern is capable of producing a single 

 prothallium. The single egg at the bottom of each arche- 

 gonium is capable of developing a single fern, and since there 

 are several archegonia on each prothallium, a prothallium is 

 thus theoretically able to produce several ferns. Usually, 

 however, only one of the eggs becomes fertilized by a sperm, 

 therefore only a single fern develops from a prothallium. Some- 

 times two eggs may grow, and occasionally three may develop, 

 so that two or three little ferns may sometimes be found grow- 

 ing from a single prothallium. 



Alternation of Generations in a Flowering Plant. In a 



