Environment and Adaptation 223 



type of reproduction in many of the lowest organ- 

 isms. Usually, however, there are developed more 

 or less specialized cells, whose sole function is repro- 

 duction. In many algae these reproductive cells es- 

 cape from the parent cell and become free-swimming 

 zoospores, which settle down and grow into new 

 plants directly. The advantage of the motile condi- 

 tion for the distribution of the species is evident 

 enough. 



Sudden changes in the environment may act 

 as powerful stimuli in inducing the formation 

 of reproductive organs. It has been shown that 

 algae in running water grow vigorously, but seldom 

 or never develop their reproductive cells; but when 

 transferred to still water they will often develop zo- 

 ospores in very great numbers. Some algae which 

 live in the air will quickly form zoospores on trans- 

 ferring them to water. Light, temperature, changes 

 in the food all of these act as stimuli in controlling 

 the reproductive processes. Thus in the water-net 

 (Hydrodictyon), plants grown in a solution of cane 

 sugar will produce an enormous number of gametes. 



While in many of the lower plants reproduction 

 is purely non-sexual, most algae produce some 

 form of gametes, or sexual cells. These are evi- 

 dently modifications of originally non-sexual cells, 

 or zoospores, and it is sometimes impossible 

 to certainly distinguish between the two. While 

 usually there is a fusion of the gametes, they 

 may develop without union under certain conditions, 



