PAIRS OF CHARACTERS 127 



cases the recessive character appears to be as much 

 due to the presence of something as the dominant 

 one — greenness as much as yellowness, wrinkledness 

 as much as roundness, and so on; and it appears 

 that in these seven cases we really are dealing with 

 seven genuine pairs of two equally real characters, 

 and that in no case is one of the characters merely 

 the absence of its counterpart. But we shall see 

 that not only can the presence and absence hypothesis 

 be made to apply to these cases, but that it probably 

 throws much light on the real nature of these 

 characters. 



The application of this theory to the characters 

 of the cotyledons does not consist merely in the 

 statement that wrinkledness is simply the absence 

 of roundness. That would be a mere juggling with 

 words. The application of this theory consists in 

 the statement that wrinkledness is due to the absence 

 of that which determines roundness. And this raises 

 the question : What is it that determines roundness ? 

 To answer this question it is necessary to consider 

 the part played by the cotyledons in the economy 

 of the plant. They are, as the reader is probably 

 aware, storehouses of food material on which the 

 young plant depends almost exclusively from the 

 moment at which germination begins until the young 

 plant has attained a height of about six inches. 



This food material is, of course, laid down in the 

 cotyledons whilst the seed is still in the pod. It 

 first appears in the form of a sugary solution, and 

 as the seed matures, this sugar is gradually converted 



