On the assumption that there exists a repulsion between the factors for erect standard and blue in a plant which is heterozygous for both, this peculiar case receives a simple explanation. The constitutions of the erect red and the hooded purple are EEbb and eeBB respectively and that of the F_1 erect purple is EeBb. Now let us suppose that in such a zygote there exists a repulsion {91} between E and B, such that when the plant forms gametes these two factors will not go into the same gamete. On this view it can only form two kinds of gametes, viz. Eb and eB, and these, of course, will be formed in equal numbers. Such a plant on self-fertilisation must give the zygotic series EEbb + 2 EeBb + eeBB, _i.e._ 1 erect red, 2 erect purples, and 1 hooded purple. And because the erect reds and the hooded purples are respectively homozygous for E and B, they must thenceforward breed true. The erect purples, on the other hand, being always formed by the union of a gamete Eb with a gamete eB, are always heterozygous for both of these factors. They can, consequently, never breed true, but must always give erect reds, erect purples, and hooded purples in the ratio 1 : 2 : 1. The experimental facts are readily explained on the assumption of repulsion between the two {92} factors B and E during the formation of the gametes in a plant which is heterozygous for both. Other similar cases of factorial repulsion have been demonstrated in the sweet pea, and two of these are also concerned with the two factors with which we have just been dealing. Two distinct varieties of pollen grains occur in this species, viz. the ordinary oblong form and a rather smaller rounded grain. The former is dominant to the latter.[7] When a cross is made between a purple with round pollen and a red with long pollen the F_1 plant is a long pollened purple. But the F_2 generation consists of purples with round pollen, purples with long pollen, and reds with long pollen in the ratio 1 : 2 : 1. No red with round pollen appears in F_2 owing to repulsion between the factors for purple (B) and for long pollen (L). Similarly plants produced by crossing a red hooded long with a red round having an erect standard give in F_1 long pollened reds with an erect standard, and these in F_2 produce the three types, round pollened erect, long pollened erect, and long pollened hooded, in the ratio 1 : 2 : 1. The repulsion here is between the long pollen factor (L) and the factor for the erect standard (E). {93}