6o Plant Genetics 



only when the enzyme is present to oxidize the chromo- 

 gen. Enzymes are very sensitive; their activities may 

 be affected or completely checked by various agents. 

 Suppose that / of the diagram be such an agent and the 

 necessary mechanism is apparent. When / is present 

 R is paralyzed, so that it cannot oxidize C. 



3. Supplementary factors. — ^A supplementary fac- 

 tor is one that is added to a dissimilar factor, with the 

 result that a character is modified in some way. 



In his work upon red-grained races of corn East 

 found occasionally a few purple grains. His conception 

 of the situation is as follows (3). The 

 pure red plant contains two comple- 

 mentary factors, one (C) a chromo- 

 gen, and the other (R) an enzyme, 

 which when brought together pro- 

 duced the red color. The purple 

 grains, however, must be explained 

 Fig. 12 by the presence of still another factor 



(/*), the resulting situation being 

 represented in fig. 13. 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 variables. For a similar reason we 

 will assume that the absence of / is constant. The 

 fiigure shows three possibilities, from which the following 

 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. 



This is a typical case of a supplementary factor, that 

 is, one which is added to a dissimilar factor, with the 



