80 HEREDITY [ch. 



Hitherto in discussing the interaction of distinct 

 pairs of factors (allelomorphs), colour alone has been 

 considered, but cases are known where colour and 

 a structural character are interdependent in the 

 same way. Interesting examples of this are known 

 in stocks and primulas. When a certain smooth- 

 leaved cream-flowered stock is crossed with a smooth 

 white, the F 1 plants are purple and hoary, i.e. they 

 revert to the ancestral wild purple and hoary-leaved 

 stock. The purple colour appears for the same reason 

 that the two forms of white sweet-pea gave purple ; 

 one colour-factor is introduced by the white parent 

 and its complement by the cream 1 . But the hoari- 

 ness appears because the parents contain a factor for 

 hoariness, which can only take effect in plants ivith 

 purple flowers. The parents are therefore smooth 

 although they contain the hoary factor. When the 

 F x hoary purples are crossed together, the F 2 genera- 

 tion consists of purple, white and cream-flowered 

 plants in the expected proportions, but only the 

 purples are hoary. Smooth-leaved purple strains do 

 exist, but these are plants lacking the hoary factor 

 altogether ; if it were present, it would appear when- 

 ever the flowers contain purple sap. 



1 The cream-colour is due to a quite distiuct factor, and the 

 pigment is in special bodies (plastids) in the cells of the petals. 

 The purple colour is due to a pigment dissolved in the sap, and, 

 is independent of the cream plastid-colour. 



