28 THE METHODS AND 



show all the possible combinations of the 

 four different factors and their absences 

 which the genetic constitution of the plant 

 can make. 



Note that one of the combinations we 

 expect to find is missing. There are white 

 erect and white hooded — white because they 

 are lacking one or other of the comple- 

 mentary ingredients necessary to the pro- 

 duction of pigment. There are purple erect 

 and purple hooded, of which the purple erect 

 must perforce contain all the four factors, 

 and the puq^le hooded must similarly con- 

 tain all of them except that for erectness. 

 But when we turn to the red class we are 

 surprised to find that they are all erect, none 

 hooded. One of the possible combinations 

 is missing. If you examine this series of 

 facts you will find there is only one possible 

 interpretation : namely that the ingredient 

 which turns the flower purple — alkalinity. 



