CHAPTER XLI. 

 SOME PRINCIPLES OF PLANT BREEDING. 



648. Object of plant breeding. Plant breeding has for its 

 object the improvement of cultivated varieties of plants, the 

 selection and improvement of promising wild plants, and the pro- 

 duction of new and better varieties. Its success is dependent to 

 some extent on a knowledge of the laws and factors of evolution, 

 as well as on an intelligent analysis and handling of the materials. 

 The factors are the same as those operating in nature on wild 

 plants, but in many points artificial methods are introduced to 

 replace the natural methods operating in the world at large. In 

 this way progress is more rapid because favorable conditions can 

 be provided, a better food supply can be furnished, competition 

 can be removed, variation can be amplified, and an intelligent 

 selection can fix upon those characters of greatest value for the 

 purpose in view, and quickly eliminate the undesirable forms. 

 Two principal lines of plant breeding might be mentioned 

 here. 



First, the improvement of existing species or varieties in one or 

 several directions, in order to obtain the highest percentage in 

 content of certain desirable substances, as starch content, sugar 

 content, oil content, protein content, etc. This results in the 

 production of what are called races. Such races can only be 

 maintained by continued selection in breeding, otherwise they 

 rapidly deteriorate. 



Second, the production of entirely new forms of plants, in the 

 nature of varieties, subvarieties, subspecies, or physiological 

 species, forms which are different from the parent by the posses- 

 sion of certain characters not shown by the parent. 



