26 



PRINCIPLES OF PLANT BREEDING 



should be familiar with the pedigree or history of the types he is 

 using. (4) He may have occasion to depart from the fixed rules 

 or principles of breeding and should then be wise in his judgment. 

 (5) It is usually best that he should keep in mind the economic 

 aspect of the results. 



Adaptation to Surroundings. The student will understand 

 that natural conditions are the most important influences involved 



FIG. 18. Breeding plants to resist root disease, 

 field under cloth. 



Diseased and resistant tobacco plants in 

 (U. S. D. A.) 



in variation and selection, whether selection be by nature or by 

 man. De Candolle studied the influences of climate upon plants 

 and reached conclusions which are very valuable in agricultural 

 practice. Plants suited to a moist climate are usually not thrifty 

 in an arid region. Corn grown in a warm climate, as in Mexico, 

 will not mature its seed in a region several hundred miles farther 

 north. The orchardist well knows that he must select the varieties 

 of apples, peaches, plums or other fruits which are adapted to his 



