48 MENDELISM CHAP. 



the same constitution as the original two whites from 

 which the experiment started. On the other hand, the 

 cross between a white of the constitution aabb and any 

 other white can never give anything but whites. For no 

 white contains both A and B, or it would not be white, 

 and a plant of the constitution aabb cannot supply the 

 complementary factor necessary for the production of 

 colour. Again, two whites of the constitution Aabb and 

 aaBb when crossed should give both coloured and white 

 flowers, the latter being three times as numerous as the 

 former. Without going into further detail it may be 

 stated that the results of a long series of crosses between 

 the various F 2 whites accorded closely with the theoretical 

 explanation. 



From the evidence afforded by this exhaustive set of 

 experiments it is impossible to resist the deduction that 

 the appearance of colour in the sweet pea depends upon 

 the interaction of two factors which are independently 

 transmitted according to the ordinary scheme of Mende- 

 lian inheritance. What these factors are is'still an open 

 question. Recent evidence of a chemical nature in- 

 dicates that colour in a flower is due to the interaction of 

 two definitive substances : (i) a colourless "chromogen," 

 or colour basis; and (2) a ferment which behaves as 

 an activator of the chromogen, and by inducing some 

 process of oxidation, leads to the formation of a coloured 

 substance. But whether these two bodies exist as such 



