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^ 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 \ But the hoari- 

 ness appears because the parents contain a factor for 

 hoariness, which can only take efiect in plants with 

 purple, flowers. The parents are therefore smooth 

 although they contain the hoary factor. When the 

 Fx hoary purples are crossed together, the F^ 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. 



^ The cream-colour is due to a quite distinct 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. 



