CHAPTER XV 

 PLANT BREEDING 



THIS very important branch of botany which is attracting 

 so much attention at the present time and which is so often 

 referred to as a new subject, no doubt began with the earliest 

 dawn of civilization. Man selected and cultivated the plants 

 best suited to his purpose and the excellent qualities of many 

 of our economic plants are no doubt largely due to plant selec- 

 tions conducted by many generations who did not fully appre- 

 ciate just what they were doing. Plant breeding is based on three 

 laws of plant growth: variation, mutation and hybridization. 

 (Fig. 92.) 



Variation is that law of nature which leads to differences 

 of more or less importance among plants of the same species. 

 Many thousands of plants of a species may appear to be the 

 same, but a close examination will show that no two are exactly 

 the same. By selecting seed from the individual plants which 

 possess the desired characters through a number of generations 

 it is frequently possible to emphasize these important charac- 

 ters. In the beginning of the sugar-beet industry in America, 

 the plants yielded about seven per cent sugar, but by careful 

 selection we now grow crops that yield double this amount. 

 Many of our most important varieties of fruits, vegetables and 

 grains are the results of more or less careful selection through 

 many generations. 



Mutation is the law by which plants produce new species 

 with well-defined characters in a single generation. The per- 

 centage of new species is very small and yet of sufficient im- 

 portance to be of considerable value in agriculture. The study 



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