HEREDITY AND VARIATION IN PLANTS 145 



The wild ancestor of our common cabbage (Fig. 23) and 

 its relatives, kale, kohlrabi, brussels sprouts, and cauliflower 

 illustrate the horticultural development by mutations. 



Mutations are very important in plant improvement, but 

 in nature they are seldom noticed. For this reason many 

 experiments were made to find artificial methods to increase 

 their number. Today, we may multiply the number of 

 mutations in many cases by a thousand times with artificial 

 treatments. X-ray treatment of seeds or growing points was 

 one of the first methods employed. Now methods more 

 nearly available to everybody are fully as successful. Ex- 

 treme cold, controlled heat, old age, and numerous chemical 

 treatments are used extensively. Another type of change, 

 polyploidy, will be considered later in this chapter. 



Johansen thought that by always selecting individuals of 

 a desirable extreme character, a new strain could be de- 

 veloped, but he found that these selections always varied 

 back toward the original or normal form. He selected small 

 beans and large ones, but found in both cases that their aver- 

 age size was not the size of the parents, but the average 

 tended toward the normal average. He thought his beans 

 might have a mixed ancestry, and therefore he bred nineteen 

 pure lines. A pure line is the progeny of a single self-fertil- 

 ized homozygous individual. The results for the pure lines 

 with either large or small beans proved to be the same as for 

 the beans that were not of a pure fine. 



The pure line technique has become very important in 

 much of the plant breeding since it produces individuals 

 more nearly uniform, which can be used as checks for com- 

 parison with the modified strain. In careful work pure lines 

 are also used for crossing. 



Variations due to the environment will not breed "true" 

 as was true for variations caused by an internal or chromatin 

 change. Large or productive plants in a good environment 

 will lose these desirable characters under unfavorable con- 



