XI SEX - 121 



dioica 9 x dioica S gave 9 5 and S S 



„ X alba $ „ S 9 only 



alba 9 x dioica ^ „ 9 9 and S S . 



The point of chief interest lies in the striking difference 

 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 fe- 

 male produces equal numbers of ovules with and without 

 the female factor, while this factor is absent in all the 

 pollen grains. Alba^ X dioica S gives the same result as 

 dioica $ X dioica S , and we must therefore suppose that 

 alba produces male and female ovules in equal numbers. 

 AlbaS X dioica^, however, gives nothing but females. 

 Unless, therefore, we assume that there is selective fer- 

 tilisation we must suppose that all the pollen grains of 

 alba carry the female factor — in other words, that so far 

 as the sex factors are concerned there is a difference be- 

 tween the ovules and pollen grains borne by the same 

 plant. Unfortunately further investigation of this case 

 is rendered impossible owing to the complete steriHty of 

 the Fi plants. 



