198 MENDELISM 



factor for the colour in question, or perhaps more 

 properly upon the absence of the proper strengthening 

 factor which converts Picotee into purple, and tinged 

 white into Painted Lady. 



The following explanation of the result so far 

 described has now been well established by further 

 experiment. In the first place, we may consider all the 

 coloured forms together as a single group opposed to 

 white. It is now clear that the coloured type of F : is 

 due to the meeting together of two factors, one of them 

 born by one white parent and the other by the second, 

 and it is necessary for both these factors to be simul- 

 taneously present in order that any colour may make 

 its appearance. We may call these two factors C and 

 R, denoting the absence of either by c and r respectively. 

 By the simple Mendel ian behaviour of these two pairs 

 of factors C-c and R-r, the ratio of nine coloured 

 plants to seven white appearing in F x is readily 

 explicable, and the way in which this happens is shown 

 in the diagram on the opposite page. 



To explain the presence of the four different types of 

 coloured plants which make their appearance in F 2 , 

 two further pairs of allelomorphs are called in. The 

 dominant member (B) of one of these, when present 

 in combination with C and R, produces the purple 

 or Picotee colour (blue), whilst its absence (b) in pre- 

 sence of C and R is accompanied by the appearance 

 of the red colours Painted Lady and tinged white. 



Purple Invincible and Painted Lady are regarded as 

 intensified forms of Picotee and tinged white respec- 

 tively. The presence of the second factor (T) is attended 



