60 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 I 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- 





 







I 



duced the red color, The purple 

 grains, however, must be explained 

 FIG. 12 by the presence of still another factor 



(P), 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 7 is constant. The 

 figure 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 



