382 



EVOLUTION, GENETICS, AND EUGENICS 



In his work upon red-grained races of corn East found occasionally 

 a few purple grains. His conception of the situation is as follows. 

 The pure red plant contains two complementary factors, one (C) a 

 chromogen, and the other (R) an enzyme, which when brought 

 together produced the red color. The purple grains, however, must 

 be explained by the presence of still another factor (P), the resulting 

 situation being represented in Fig. 73. Of course when C is absent 

 no pigment whatsoever can be produced. As a consequence we will 

 assume that the presence of C is constant, and that P and R are vari- 

 ables. For a similar reason we will assume that the absence of I is 

 constant. The figure shows three possibilities, from which the follow- 

 ing conclusions may be drawn: (i) when P and R are both present 

 the result is purple grains; (2) red appears only in the absence of P; 

 (3) P although present will not develop any color in the absence of R. 



Puiple 



Red 



White 



Fig. 73. — Diagram illustrating action of supplementary factor. (From 

 Coulter and Coulter.) 



This is a typical case of a supplementary factor, that is, one which 

 is added to a dissimilar factor, with the result that the color character 

 is modified. The mechanism of this situation will make clearer the 

 behavior of the supplementary factor. If C is the chromogen and R 

 the enzyme, what is P? The suggested answer can be obtained 

 from plant chemistry. It is found that the purple pigment is produced 

 by the same substance as the red, but represents a higher state of 

 oxidation. The conclusion is obvious. C is oxidized by i? up to a 

 certain point, where red is produced; but if P is also present it repre- 

 sents an additional enzyme, which attacks the red pigment and oxidizes 

 it still further into purple. P is incapable of attacking the original 

 chromogen, but when R carries the attack to a certain point, P can 

 function and carry the oxidation further. As a consequence P without 

 R gives white grains, while R gives red grains only in the absence of P. 



