338 THE BIOLOGY OF FLOWERING PLANTS 



The mode of determination of the different types of indi- 

 vidual in the more complex cases of distribution have not 

 been elucidated. 



§ 7. Changes in Sex Distribution in the Course of 



Evolution 



' The changes in the mode of sex distribution during the 

 evolution of the land flora are of great interest. In the 

 ferns proper the gametophyte is usually hermaphrodite, 

 though this condition is easily modified. The horsetail 

 gametophyte is dioecious. With the advent of heterospory 

 dioecism in the gametophyte generation became fixed, 

 and this has remained unaltered through all subsequent 

 changes. Heterospory was probably at first united with 

 bisexuaHty of the individual, as in the heterosporous 

 pteridophytes which still exist. These are not primitive 

 forms, and it is possible that in other lines heterospory may 

 have been early associated with a separation of the sporophyte 

 individuals into two classes, male and female. In the 

 earliest seed plants, the pteridosperms, there may have 

 been only one type of sporophyte. From these may have 

 arisen, on the one hand the dioecious cycads, and on the 

 the other the mesozoic Bennettites with its hermaphrodite 

 flower. At the end of some such evolutionary line come 

 the angiosperms. The gymnosperms are all diclinous, but 

 about 75 per cent, of existing angiosperms have herma- 

 phrodite flowers. The hermaphrodite flower is characteristic 

 of the primitive angiosperms and may be taken as primitive 

 for the group. In the course of further evolution, how- 

 ever, it has undergone, more than once, modification 

 towards unisexuality, which is pronounced, if not rigidly 

 fixed, in a great many flowering plants. It is very likely 

 that the great range of intermediate conditions represent 

 stages in the trend towards complete and simple dioecism. 



Beginning, therefore, with a union of the two sexes in 

 both gametophyte and sporophyte, the initial evolutionary 

 change was the separation of male and female gametophytes. 



