xiii CERTAIN COMI'LICATKjNS i 



3 J 



and alba. Males a|)i)car when diinj is used as the 

 female parent but n(jt when the female dioiiii is 

 crossed b\- male alba. It is [)ossible l(^ •^I'^l^^'-^'t 

 more than one scheme to cover these fads, but wc 

 ma)' confine ourselves here to that which seems 

 most in accord with the general trend of other 

 cases. W'e will suppose that in (iioica fcmalcncss 

 is dominant to maleness, and that the female is 

 heteroz\-gous for this additional factor. In this 

 species, then, the female [)ruduccs etjual numbers (;f 

 ovules with and without the female factor, while 

 this factor is absent in all the pollen i^rain^. Albal- 

 X dioica gives the same result as dioica ? x 

 dioicao ^ ?i\M\ we must therefore suppose that idba 

 produces male and female ovules in ec[ual numbers. 

 Alba 6 X dioica 9 , however, gives nothini^ but females. 

 Unless, therefore, we assume that there is selective 

 fertilisation we must suppose that all the pollen 

 grains of a/ba carry the female factor — in other 

 words, that so far as the sex factors are concerned 

 there is a difference between the ovules and i>»llen 

 grains borne b\' the same plant. Unfortunaiel\- 

 further investigation of this case is rendered im- 

 possible owing to the complete sterility of the l*' 

 plants. 



That the ^possibility of a difference between the 

 ovules and pollen grains of the same individual must 

 be taken into acci^unt in futuie wt^rk' 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 fr( ni s[Kxial strains o( the single 

 variet}'. " You must understand withall," wrote 



