KNOWLEDGE OF HEREDITY IN PISUM. 97 



describe in words. The same shapes may sometimes be 

 recognized again in the later generations from a cross, and are 

 generally uniform upon the same plant, thus showing some 

 analogy with the indent character. It is thus probable that 

 other pairs of allelomorphs concerned with the shape of seeds 

 show segregation in addition to the two pairs referred to above. 

 But it is not easy to obtain proof of this in the case of 

 characters so difficult of definition. 



Some of these characters may depend upon pressure in the 

 pod and upon other similar factors. 



(iv.) Characters not inherited. — Endless differences in the 

 shape of individual seeds are associated with such accidental 

 circumstances as position in the pod, fullness of the pod, the 

 position of the pod on the plant, and many others. 



All the seeds of the same pod sometimes show an unusual 

 character, for instance, pitting in the case of a round pea. Or 

 a pod may contain a row of seeds much smaller than the normal. 

 The offspring of such seeds are not to be distinguished from 

 those arising from normal seeds of the same plant. 



B. — Internal Characters. 



Gregory (10) showed that peas belonging to the round and 

 indent classes have similar starch grains which are large and 

 single, whilst the starch grains of wi'inkled peas are small and 

 compound. ^ 



Darbishire (6) has recently observed that the seeds of 

 heterozygotes from the cross round X wrinkled, though 

 showing the dominant character externally, are intermediate 

 as regards the character of their starch grains. 



2. — Colour of the Cotyledons. 



In the majority of cases yellow and green are sharply dis- 

 continuous. There is, however, a tendency for normally green 

 seeds to turn gradually yellow, especially if kept in the light, 

 and this tendency is much more marked in some strains of peas 

 than in others. In some cases, and especially in bad weather, 

 this tinging may be carried so far that many seeds in the earliest 

 ripeniag pods of a green-seeded plant may have become 

 more or less distinctly yellow before the upper pods are ripe. 



