SEX 



1 1 1 



either male or female, though both are borne on the 

 same individual. In others, again, the conditions are 

 even more complex, for the same plant may bear 

 flowers of three kinds, viz. male, female, and herma- 

 phrodite. Or it may be that these three forms 

 occur in the same species but in different individuals 

 female and hermaphrodites in one species ; males, 

 females, and hermaphrodites in another. One case, 

 however, must be mentioned as it suggests a possi- 

 bility which we have not hitherto encountered. In 

 the common English bryony (J3ryonia dioica) the 

 sexes are separate, some plants having only male 

 and others only female flowers. In another Euro- 

 pean species, B. alba, both male and female flowers 

 occur on the same plant. Correns crossed these 

 two species reciprocally, and also fertilised B. dioica 

 by its own male with the following results : 



dioica ? x dioica $ gave $ ? and 6 3 



x alba (J $ $ only 

 alba ? x dioica $ $ ? and c? <. 



The point of chief interest lies in the striking differ- 

 ence shown by the reciprocal crosses between dioica 

 and alba. Males appear when alba is used as the 

 female parent but not when the female dioica is 

 crossed by male alba. It is possible to suggest 

 more than one scheme to cover these facts, but we 

 may confine ourselves here to that which seems 

 most in accord with the general trend of other 

 cases. We will suppose that in dioica femaleness 

 is dominant to maleness, and that the female is 

 heterozygous for this additional factor. In this 

 species, then, the female produces equal numbers of 



