EVOLUTION OF SEX DISTRIBUTION 339 



Then separation of sexual characters in the sporophyte took 

 place, at first Hmited to the spore, spreading to the sporangia, 

 sporophylls, and finally, on the lines which lead to the 

 various gymnosperm families, to the cones or even to the 

 whole plant, as in the cycad type of pronounced dioecism. 

 A critical change was the evolution of the hermaphrodite 

 flower, from which modern diclinous species of angiosperms 

 have been derived by suppression. We must, however, 

 mention the theory of von Wettstein, that the primitive 

 angiosperms are to be looked for in the Cupuliferae, a 

 group with marked dicliny, a theory which has not found 

 very general acceptance. 



Such a history must leave us in doubt with regard to the 

 fundamental importance of cross fertilisation or out- 

 breeding, which is most easily and decisively secured by 

 dicecism. When sex distribution has undergone so many 

 changes during the course of evolution it appears unlikely 

 that any particular type has great superiority. Either cross 

 fertilisation is not of great importance, or it is secured with 

 sufficient frequency, no matter how the sexes are combined 

 or separated. The importance of sex is, however, emphasised 

 by its maintenance through this long and varied history. 

 Parthenogenesis and apogamy crop up here and there, but 

 have never been permanently established. 



§ 8. Secondary Sex Characters 



In connection with this discussion of sexual reproduction 

 reference may be made to a phenomenon of great interest 

 in the animal kingdom, though relatively quite unimportant 

 among plants, the occurrence of secondary sexual characters. 

 Every one knows the secondary diff"erences between a cock 

 and a hen, but it is not easy to get striking differences between 

 male and female plants, apart from the difference between 

 stamens and ovaries. It is easy to pick out a male or a 

 female willow at a distance, but what catches the eye is the 

 brilliant yellow of the stamens due to the presence of pollen. 



A true secondary distinction has already been touched 



