THE ORIGIN OF SEX IN PLANTS 17 



zygote then rounds off, forms a cell-wall, and presently 

 may germinate into a new plant. Here then is a primitive 

 distinction of sex, which is, however, only functional. 

 Structurally the sexual cells are isogametes ; but function- 

 ally the female may be recognised by its early loss of 

 motility, and by the attraction which it exercises over 

 the motile males. 



VIII 



FIG. 8. 



Ectocarpus secundus. i = filament bearing male ( $ , and female ($) gametangia ; 

 ii = male gametes; iii = female gametes ; iv-vi = stages of fertilization ; vii-viii= 

 zygotes. (After Sanvageau.) 



The next step is a distinction in size between the two 

 types of gamete, and it is seen in another species of the 

 genus Ectocarpus, viz. E. secundus. In this plant the 

 partitioned sporangia are of two different kinds, which 

 are borne on the same individual. One is small-celled, 

 and gives rise to small male gametes (Fig. 8, i, $, ii) ; the 

 other is larger-celled, and gives rise to larger female 

 gametes (Fig. 8, i, ?, iii). Both kinds of gametes are, 

 however, motile, and have the form usual in the Brown 



