THE PLANT KINGDOM 



247 



given up, or resorted to only under special conditions. The 

 sexual cells themselves became further differentiated into two 

 sorts — small, active ?na/e gametes or sperms and larger, non- 

 motile female gametes or eggs, a condition which now accompanies 

 sexuality so commonly that instances of equal gametes are corn- 



el B 



Fig. 137. — The beginnings of sexuality. A very simple alga, Chlamydomonas, 

 (A) in which two cells may be differentiated as gametes and unite with each other. 

 From their union a group of new individuals arises. In the case of fertilization 

 here illustrated {B) the gametes are slightly different in size, foreshadowing the 

 development of male and female gametes. 



paratively rare. The causes which led to the development of 

 sexual reproduction are unknown, but the process is so nearly 

 universal, not only among plants but throughout the animal 

 kingdom, that we are forced to believe it must have some special 

 significance. There is evidence that sexual fusion results in 

 increased vigor, particularly when the two gametes come from 

 different individuals; but we also know man}- plants which may 

 reproduce indefinitely by various asexual processes without evi- 

 dent loss in vitality. However that may be, a considerable 



