THE EVOLUTION OF SEX 



223 



G and S standing for the gametophyte and sporophyte gener- 

 ations, respectively. It will appear later that the higher plants 

 have essentially the same life history formula as this. 



248. The evolution of sex. The account of the algae has 

 given material for a brief discussion of the origin and evolution 

 of sex. It has been shown that the simplest forms of sexual 

 cells, or gametes, have essentially the same structure and origin 

 as the zoospores. These conditions are illustrated by Sphcerella, 

 Ulothrix, Ulva, and Cladophora. The difference between the 

 gamete and zoospore is chiefly one of size. The gametes are 

 smaller because they are generally formed more numerously in 

 their mother cells. Gametes sometimes are able to germinate 

 like zoospores, but such gametes are 

 apt to develop small and weak plants, 

 and as a rule they must fuse with 

 one another in pairs in order to live. 

 It seems clear that sex arose with 

 the development of a type of zoospore 

 smaller and apparently weaker in its 

 power of vegetative growth than the 

 normal zoospore. These smaller zoo- 

 spores formed the habit of fusing in 

 pairs, and this habit, finally becoming 

 fixed in the plant's life history, de- 

 veloped into a method of sexual repro- 

 duction. 



After the establishment of sex in a FlG - 205 - Criteria multiftda 

 group of plants, further developments A > the lar s e female gamete; 



n \ j i. j-c J.T- * ^ B < the 8ame at rest and sur - 



will tend to modify the form of the 



gametes, the process finally ending in 



their differentiation into eggs and 



sperms. The simplest gametes are so similar in form and size 



that they cannot be distinguished as male and female, but 



a number of algse have gametes which are different in size, 



although similar in structure, or morphology. This condition 



rounded by small male ga- 

 metes, one of which, m, is 

 shown in the act of fusion 



