CHAPTER III 



THE task that Mendel set before himself was to gain 

 some clear conception of the manner in which the definite 

 and fixed varieties found within a species are related to 

 one another, and he realised at the outset that the best 

 chance of success lay in working with material of such a 

 nature as to reduce the problem to its simplest terms. He 

 decided that the plant with which he was to work must be 

 normally self-fertilising and unlikely to be crossed through 

 the interference of insects, while at the same time it must 

 possess definite fixed varieties which bred true to type. 

 In the common pea (Pisum sativum) he found the plant 

 he sought. A hardy annual, prolific, easily worked, 

 Pisum has a further advantage in that the insects which 

 normally visit flowers are unable to gather pollen from it 

 and so to bring about cross fertilisation. At the same 

 time it exists in a number of strains presenting* well- 

 marked and fixed differences. The flowers may be purple, 

 or red, or white ; the plants may be tall or dwarf ; the 

 ripe seeds may be yellow or green, round or wrinkled - 

 such are a few of the characters in which the various races 

 of peas differ from one another, 

 c 17 



