II] 



structural Characters : Plants 



29 



shape of the grains, and their simple or compound nature, may be 

 governed by distinct factors. He regards the absorptive power as again 

 separable from these features. 



A'OU 



tXJO^l 





Round and Indent. 



Wrinkled. 



Fig. 9. Outlines of starch grains in the different types of peas. The 

 wrinkled contains many compound grains. (From Gregory.) Magni- 

 fication the same in both figures. 



A third type of pea, of which the purple sugar-peas {sans parchetnin) 

 are a good instance, may be described as "indent." These also have 

 large, simple starch grains. Such seeds are of irregular flattened shape and 

 may be confounded with true wrinkled peas. Their properties are entirely 

 different, and the two sorts must be carefully distinguished. One of their 

 special properties will be discussed in a later chapter, but here it must 

 suffice to say that their genetic properties are essentially those of round 

 peas. Much confusion has been introduced by want of care in distinguish- 

 ing these types. 



Intermediates, which on casual sorting, cannot be classed either as 

 round or wrinkled, sometimes occur. Some of the round types {e.g. 

 Victoria Marrow) contain a large proportion of such seeds. Their 

 peculiarity is almost certainly due to environmental influence, though 

 obviously the liability to this affection may be transmitted. When such 

 seeds are found in F^ from a cross between thoroughly round and wrinkled 

 varieties, the pitting, when it exists, generally affects all the round seeds of 

 the/^^5- in which it occurs. With experience such pitting can immediately 

 be recognized as distinct from the true genetic wrinkling, and in our 

 experiments the results of a further sowing have repeatedly confirmed the 

 judgment made by inspection of the seeds. 



A complete account of all the phenomena would run to great length. 

 The interrelations of round and wrinkled seeds are to be recommended as 

 offering perhaps the most favourable example for an investigation of the 

 chemical nature of a genetic factor. The wrinkling is evidently the 

 consequence of a particular method of drying, and this must depend on the 

 nature of the reserve-materials. A first step would be to determine the 

 relative amounts of sugar and starch in the two chief types. It is natural 



