138 Colours in Critical Parts [CH. 



Observation, for example, gave 70 : 2 1 : 36, the expectation 

 being 72:24:32. In certain other cases this resolution- 

 effect did not occur, though from analogy it might have 

 been expected. Toyama regards the distinction as due to 

 differences in the whites used, but it seems not impossible 

 that it was really the yellows which possessed individual 

 differences in this case. In either event there are difficulties 

 to be faced, and on the evidence it is not clear which account 

 is actually the more probable. 



There are some illustrations of a principle by which the 

 colour of one part of the organism may limit or control the 

 possible colours of other parts. 



In animals it is fairly certain that the eye-colour may act 

 in this way, certain coat-colours being produced only if the 

 eye be black, and others only if the eye be chocolate, but 

 the facts are still somewhat obscure. 



If the stem of the Chinese Primula be green and not red 

 the deeper flower-colours cannot be developed in self-coloured 

 types. A cross with a red-stemmed type, however pale in 

 flower-colour, at once reveals the presence of the factors for 

 the deep colours if they are there. 



On -the contrary, the white-edged types, such as Button's 

 "Sirdar," though their flowers may be of a deep shade of 

 purple or red, appear exclusively on stems which are green 

 throughout except for a development of red colour at the 

 collar or extreme base of the petioles. Such "Sirdars" 

 cannot exist on a wholly-coloured stem. The stem may be 

 parti-coloured in Primulas though the flower is whole- 

 coloured, but these special types of parti-coloured flower 

 can only occur on a parti-coloured stem. In the /% series 

 it is curious to see these deeply coloured, white-edged, 

 flowers on stems apparently green, while none of their 

 green-stemmed sisters with self-coloured flowers can bear a 

 flower darker than pale salmon-pink. 



Another striking example of the same phenomenon is to 

 be seen in these Primulas, with the difference that there the 

 want of a particular colour in the critical or "controlling" 

 position is due to the dominance of a negative character, not 

 to the absence of a complementary one. Certain deep red 

 spots occur in some varieties, e.g. Sutton's "Crimson King" 



