DEVELOPMENT OF PLANTS 



159 



history very similar to that of Sphaerella but the formation of 

 the gametes reveals a variation that, gives us an understanding 

 of how these bodies came to differ and finally became distin- 

 guishable as male and female gametes. In some of the species 

 of Chlamydomonas, to be sure, the gametes are similar and can 

 not be distinguished as male or female (Fig. 101, B). This 

 stage of sexuality is called isogamous, meaning similar gametes. 

 In other species, there is a decided variation in the size of the 

 gametes, certain cells or plants producing large and less active 



Fig. ioi. 



Fig. 102. 



Fig. ioi. Features in the life history of Chlamydomonas: A, character 

 of the mobile plant. B, conjugation of isogamous gametes. C, a cell 

 dividing to form numerous small male gametes. D, a plant forming two 

 large female gametes. E, male and female gametes about to conjugate. 



Fig. 102. Features in the life history of Paiidorina: A, a colony of 

 plants. B, each plant of the colony dividing to form a new colony. C , the 

 plants of a colony escaping as gametes. D, the conjugation of two gam- 

 etes of unequal size. E, later stage in the conjugation. F, gametospore 

 or resting spore. G, large zoospore formed from the gametospore. H, a 

 colony formed by the division of the zoospore, G. 



cells that are termed female gametes, while other plants produce 

 numerous minute male gametes that swim actively about (Fig. 

 101, C, D). This stage of sexuality, where the gametes differ 



