120 BREEDING CROP PLANTS 



Briefly, it consisted of growing and harvesting a 100-plant plot 

 from each plant. Selection was continued the following year. 

 The selections of most promise were increased and given exten- 

 sive trials by farmers. By this method new forms of superior 

 value were discovered. 



The pure-line method of breeding self-fertilized crops was 

 independently discovered and later adopted (1891) by the Svalof 

 experiment station in Sweden. The director of the station, H. 

 Nilsson, was led to its adoption by the accidental discovery that 

 only those plots planted with seed coming from a single plant 

 exhibited uniformity (Newman, 1912). DeVries (1907) says: 



11 To this accidental circumstance, combined with the exact scientific 

 method of keeping extensive records, the discovery of the cause of 

 the diversity of the cultures was due. For precisely those cultures 

 which were derived from one ear only were found to be pure and uniform, 

 all others offering to the eye a more or less motley assemblage of forms." 



The fact that many of the agricultural varieties grown in 

 Sweden at the present time are the result of this method of breed- 

 ing is sufficient evidence of its success. 



In addition to individual plant selection, the older mass 

 selection is' sometimes used with self -fertilized crops. Mass 

 selection is the selection of a group of individuals which seem to 

 embody the desired characters. No attempt is made to grow 

 the offspring of the different individuals separately and hence a 

 pure-line study is impossible. In spite of this fact, mass selection 

 sometimes has a place in correct breeding. For example, it may 

 be advantageous to let nature eliminate non-hardy forms of a 

 winter wheat variety before beginning a study of individual 

 plant progenies. 



SELECTION WITHIN A PURE LINE 



Early in the twentieth century Johannsen (1903, 1913) began 

 his famous experiments with beans and barley which resulted in 

 the discovery of facts which led to the development of the pure- 

 line theory. Johannsen found that selection within a pure line 

 was futile. Table XIX is typical of what he obtained by selection 

 within each of 19 different pure lines of beans. 



Since Johannsen announced his pure-line concept, several 

 investigators working with other crops and other characters have 

 verified his conclusions. 



