132 BREEDING 



do not change colour during this process. The 

 yellowing of the former is brought about by the 

 gradual fading and disappearance of the green pig- 

 ment, which thus leaves the yellow pigment (which is 

 present in both kinds) exposed. The successive 

 stages in the fading of the green can be easily observed. 

 The simultaneous presence of both green and yellow 

 pigment in yellow and in green peas has also been 

 demonstrated. 



To sum up, green-seeded varieties contain two 

 pigments in their cotyledons, a yellow and a green ; 

 neither of them fade during the process of ripening, 

 and, inasmuch as the green masks the yellow, the 

 ripe seed is green. Yellow-seeded varieties also 

 contain the same two pigments, but the green fades 

 during the process of ripening ; so that the ripe 

 seed is yellow. This fading of the green pigment 

 in the yellow pea is supposed to be brought about 

 by the ^presence of some substance which is absent 

 from the green pea. 



It is not possible to indicate the manner in which 

 this presence and absence hypothesis applies to the 

 remaining of the seven characters dealt with by 

 Mendel, except, of course, in those cases in which 

 its application is obvious — for instance, to the colour 

 of the seed-coats, which may be grey or white. It 

 would be easy to say that normality of stem was 

 due to the presence, and fasciation to the absence, 

 of some factor controlling the normal succession of 

 nodes in the stem ; but this is doing no more than 

 re-stating the problem in other terms. And until 



