56 Plant Genetics 



explanation of the behavior of complementary factors 

 in many cases of color inheritance. 



Where other characters are involved the mechanism 

 must be somewhat different. In some cases the two fac- 

 tors may be the enzyme and the compound the enzyme 

 attacks, as in the oxidase and chromogen situation just 

 described. On the other hand, we might be dealing 

 with two chemical compounds that are inert when occur- 

 ring separately but active when brought together, 

 active in such a way as to produce a distinctly new 

 character. Also two active substances might neutralize 

 one another when brought together in a hybrid, and the 

 failure in their activity might result either in a new char- 

 acter or the failure of some parental character to develop. 

 Such are some of the possible mechanisms to explain the 

 behavior of complementary factors. 



Hybridizing, therefore, is much like mixing chemicals 

 in a test tube. We know that very wide crosses cannot 

 be made successfully; but the surprising thing is that 

 certain very close crosses are constantly unsuccessful, 

 even though both parents may cross freely with closely 

 related types. We obtain a glimpse of the possibility 

 of such apparently inconsistent behavior when we 

 consider the chemical possibilities suggested by the 

 behavior of complementary factors. 



The origin of complementary factors is an interesting 

 field of speculation. Did they originate together or 

 separately? A natural inference would be that they 

 originated together, for neither would be of any use with- 

 out the other. It should be remembered, however, that 

 the idea of use as explaining the occurrence of every- 

 thing in a plant is being abandoned; one must think 



