XI SEX 117 



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 

 ovules with and without the female factor, while 

 this factor is absent in all the pollen grains. Alba ? 

 X dioica $ gives the same result as dioica ? x 

 dioica 6 , and we must therefore suppose that aida 

 produces male and female ovules in equal numbers. 

 Ali>a 3 X dioica ? , however, gives nothing but females. 

 Unless, therefore, we assume that there is selective 

 fertilisation 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 between the ovules and pollen 

 grains borne by the same plant. Unfortunately 

 further investigation of this case is rendered im- 

 possible owing to the complete sterility of the Fj 

 plants. 



That the possibility of a difference between the 

 ovules and pollen grains of the same individual must 

 be taken into account in future work there is 

 evidence from quite a different source. The double 

 stock is an old horticultural favourite, and for centuries 

 it has been known that of itself it sets no seed, but 

 must be raised from special strains of the single 

 variety. " You must understand withall," wrote 

 John Parkinson of his gilloflowers,^ " that those 

 plants that beare double flowers, doe beare no seed 



1 Paradisus Terrestris, London, 1629, p. 261. 



