PLANT BREEDING 515 



this way it was possible to combine in one variety of wheat the 

 strong stem, the closed head, and the winter-resisting qualities 

 needed for successful farming in this region. By similar methods 

 it has been possible to combine three or more characters desired 

 in a plant from as many different varieties of ancestors. 



376. Breeding for immunity. The chief problem is to get 

 organisms that combine desirable qualities and show none of 

 the undesirable qualities. It is not possible, as we have seen, 

 to transmit all the characters appearing in a combination 

 formed by hybridizing, or even in a combination resulting from 

 segregation. It is necessary that the elements from the two 

 parental gametes which determine a given character shall be 

 either both dominant or both recessive. If only one of them 

 is dominant, the particular individual may have the quality in 

 which we are interested, but its offspring will be of two kinds 

 (see Fig. 215). In the case of wheat, immunity to rust is reces- 

 sive. It is thus possible to establish strains of wheat that com- 

 bine immunity with other desirable qualities ; for, as we have 

 seen, it is only necessary to breed the hybrids into the following 

 generation in order to get a complete segregation of the various 

 qualities and their different combinations. From the third gen- 

 eration we can then select the offspring that appear in a pure 

 recessive or in a pure dominant condition.^ 



377. Practical breeding. The failure of their hybrids to breed 

 true has been the despair of plant and animal breeders in the 

 past. Only those were successful who, like Luther Burbank, 

 were patient enough to try out vast numbers of hybrids and 

 keen enough to detect the rare individuals that would breed 

 true with regard to the desirable qualities or combinations. 

 With the discoveries of the principles of heredity every intelli- 

 gent fancier or breeder can produce new varieties of organisms 

 without limit, obtaining almost any combination of useful or 

 fancy characters that he may desire. When Burbank produced 

 a "white blackberry," he did not get a plant with a new char- 



1 Of course it is not necessary to carry the test so far with plants that can 

 be propagated vegetatively. 



