SOME PRINCIPLES OF PLANT BREEDING 473 



Indian corn is a good example of this. Originally a subtropical 

 plant it is now cultivated over a wide range of the north temper- 

 ate zone, with varieties of low stature which mature in six to 

 eight weeks in the northern latitudes. A very important object 

 in plant breeding, and one which should not be overlooked, is the 

 harmonious development of all parts, the general form and con- 

 stitution of the plant, in order that these auxiliary characters 

 which contribute to the well being of the plant as a whole, which 

 bring the greatest returns in proportion to the cost of production, 

 as ease of cultivation and handling, conformity to soil and cli- 

 matic conditions, resistance to disease, etc., may be present. 

 Breeding for varieties which are resistant to disease is one of the 

 most recent efforts of the plant breeder. Its success is based on 

 the same natural laws which make possible the improvement of 

 varieties in many other directions, viz., variation and selection. 

 Plants vary in their disease-resisting qualities just as animals do. 

 It is rare that one finds all of the plants in a given locality equally 

 subject to a fungus disease. The resistant individuals are 

 selected to propagate from, either by seed or by asexual means 

 according to the case in hand. This process of selection is con- 

 tinued for several years, weeding out those which are susceptible 

 to attack until resistant varieties are obtained. Some success 

 has already been obtained in this direction in the case of rust-* 

 resisting varieties of wheat, and in varieties of cotton, clover, etc., 

 which are resistant to certain of the diseases common to them. 

 It is not to be expected that complete resistance to disease in 

 plants will be attained by plant breeding of disease-resistant varie- 

 ties, because variation cannot be eliminated. Some individuals 

 will be more susceptible than others and will now and then 

 become diseased, the percentage varying with the variations in 

 external and cultural conditions as well as with the persistence 

 and intelligence with which the selection is continued. Disease- 

 resistant varieties, like other varieties, depend on continued selec- 

 tion for the maintenance of a high standard of excellence. 



659. " Breaking the type " in plant breeding. When the 

 plant breeder undertakes to improve a variety, or develop new 



