| dioica | ♀ | × dioica | ♂ gave | ♀ ♀ and ♂ ♂ |
| " | × alba | ♂ " | ♀ ♀ only | |
| alba | ♀ | × dioica | ♂ " | ♀ ♀ and ♂ ♂. |
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 ♀ × dioica ♂ gives the same result as dioica ♀ × dioica ♂, and we must therefore suppose that alba produces male and female ovules in equal numbers. Alba ♂ 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 impossible owing to the complete sterility of the F1 plants.
Single and double stocks raised from the same single parent.
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,[[9]] "that those plants that beare double flowers, doe beare
From all this it is clear enough that there is much to be done before the problem of sex is solved even so far as the biologist can ever expect to solve it. The possibilities are many, and many a fresh set of facts is needed before we can hope to decide among them. Yet the occasional glimpses of clear-cut and orderly phenomena, which Mendelian spectacles have already enabled us to catch, offer a fair hope that some day they may all be brought into focus, and assigned their proper places in a general scheme which shall embrace them all. Then, though not till then, will the problem of the nature of sex pass from the hands of the biologist into those of the physicist and the chemist.