46 APPLICATION OF PRINCIPLES OF HEREDITY TO BREEDING. 



from the whole plot. This method would take advantage of the 

 well-known vigor which arises when two varieties of corn are crossed. 



Shiill has recently proposed a plan somewhat similar to the above, 

 and one which is really an improvement on it in one respect, but not 

 so good in another.^ He suggests using two pure strains, planting 

 one of them off by itself to get pure seed of it for the seed patches 

 the next year, while at the same time another seed patch for growing 

 field seed is planted of alternate rows of the two varieties. In this 

 patch all the stalks of the variety grown alone in the other patch 

 are to be detasseled. All the seed produced by the detasseled stalks 

 will be heterozygote, while all the seed on the remaining stalks will 

 be pure bred of the other strain from that planted alone in the 

 smaller patch. The principal difficulty with this plan is that of find- 

 ing isolated spots for two seed patches instead of for one, as in the 

 plan in which the two varieties are both planted in alternate rows 

 in one patch. In ShulFs plan the two strains could be kept prac- 

 tically pure; in the other plan they would mix to a slight extent. 



HYBRIDIZATION AND SELECTION. 



We have been considering only the effect of selection without 

 deliberate hybridization or cross-fertilization, except such as occurs 

 naturally in certain of the crops considered. We shall now consider 

 the application of artificial cross-fertilization and subsequent selec- 

 tion to the art of improving farm crops. 



As in the case of selection alone, the methods and results differ 

 for vegetatively propagated, close-fertilized, and cross-fertilized crops. 



VEGETATIVELY PROPAGATED CROPS. 



It is perhaps easier to secure the advantage of hybridization in 

 vegetatively propagated crops, such as fruits, berries, potatoes, hops, 

 and sugar cane, than it is from those crops that reproduce from 

 seed. The reason for this is that some of the heterozygote forms 

 which occur in the first generation of the hybrid may be highly 

 valuable, and these heterozygote forms can be propagated true to 

 type because they are not propagated from seed. For instance, sup- 

 pose we cross two varieties of apples or potatoes and get in the first 

 generation a plant from which can be made a valuable new variety. 

 All that is necessary is to propagate this new variety by cuttings. 



In the case of potatoes very little hybridizing has been done. 

 The seedling plants do not attain their full development until propa- 

 gated from the tubers for two or three years. It is therefore nec- 

 essary gradually to eliminate the poorer stocks and to grow for 

 some time a good many of the forms which result from crossing, to 

 see whether or not they are valuable. 



a See Report, American Breeders' Association, vol. 5. 



165 



