XI COUPLING AND REPULSION 131 



The most numerous classes are those resembling 

 the original parents, but all of the possible eight 

 types of plant are represented, though some are very- 

 rare. 



The case of what we may term the B, E, L series 

 is not the only one in which the members of a little 

 group of factors behave towards one another in this 

 peculiar way. We have already mentioned (p. 79) 

 that the factor for normal flower (iV) as opposed to 

 cretin, and that for the fertile anther i^B) as opposed 

 to sterile, behave in the same way. To these two 

 factors we may add a third. In some sweet-peas 

 the axil from which the leaf stalk springs is purple- 

 red in colour, while in others it is green.^ The 

 former, the dark axil, is dominant to the light axil. 

 The factor for dark axil D exhibits the phenomenon 

 of coupling and repulsion with either of the two 

 factors, A^ or F. The D, F, N series behaves similarly 

 to the B, E, L series, though the intensity of the 

 coupling values is widely different in the two cases. 

 It should be added that, so far as we know at present, 

 the two series are quite independent of one another. 



Since the phenomenon was discovered in the 

 sweet-pea other examples have been found both in 

 plants and animals. It has been described in 

 Antirrhinums, in Primulas, in peas, wheat, maize, 

 and rats. In no species, however, has it been 

 worked out with such wealth of detail as in the 

 pomace fly {Drosophila ampelophild) ; but the im- 



1 The axil is always green in pure' white-flowered varieties, i.e. 

 ' those whicli have light seeds. Such whites, however, may carry the 

 factor for the dark axil, and when crossed with a light axilled coloured- 

 plant the dark axil appears in their progeny. 



