IMPROVEMENT OF FIELD CROPS 



21 



Influence of crossing as a cause of variation. 



yield in grains of 100 plants, showing greater 

 variation in yield of hybrid wheat than of either 

 parent form. The yield of the hybrid is indicated 

 by the line marked — x — (After Hays.) 



42. Crossing. — Crossing two unlike forms or two varieties 

 may not be a fundamental cause of variation. Some other 

 cause must have operated to have produced the two unlike 

 forms. In practice, however, crossing is a means of inducing 

 variation, so as to enable the breeder to select forms more 

 nearly suited to his ideal. This is shown by Hays^ in the 

 case of a hybrid between Fife and Blue Stem wheat. 



Some of the plants 

 of hybrid wheat yielded 

 more and some less 

 than any of the plants 

 of either the Fife or of 

 the Blue Stem. If the 

 yield is the character- 

 istic desired, then a few 

 plants of the hybrid 

 were better than either 

 of the present varieties. 



Crossing is also employed not only to induce variation out 

 to combine two or more desirable qualities in one plant. 



43. B. Selection. — Plants having varied either through the 

 efforts of the breeder or otherwise, the next step is to select 

 plants having the characteristics desired. " Selection is the 

 surest and most powerful instrument that man possesses for the 

 modification of living organisms."^ 



The unit of selection is the individual. In the case of wheat 

 the unit is not the seed, nor even the head of the wheat, but it 

 is the stool containing several heads and many seeds which have 

 been produced from a single seed. In the case of the potato it 

 is the single hill and not the single potato. However, in plants, 

 unlike higher animals, portions may be used for the purpose of 



** Willet M. Hays. Plant Breeding. Division of Vegetable Physiologj' and 

 Pathology, U. S. Department of Agriculture, Bui. 29, p. 21. 



« Henry L. De Vilmorin. E. S. R., Vol. XI, p. 19. 



