dioica [female] x dioica [male] gave [female] [female] and [male] [male] " x alba [male] " [female] [female] only alba [female] x dioica [male] " [female] [female] and [male] [male]. 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 female produces equal numbers of ovules with and without the female factor, while this factor is absent in all the pollen grains. _Alba_ [female] x _dioica_ [male] gives the same result as _dioica_ [female] x _dioica_ [male], and we must therefore suppose that alba produces male and female ovules in equal numbers. _Alba_ [male] x _dioica_ [female], 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 impossible owing to the complete sterility of the F_1 plants. {122} [Illustration: FIG. 27. Single and double stocks raised from the same single parent.]