392 PLANT BREEDING 



tent. In com the percentage of sugar, starch, protein, 

 and oil are all important considerations for particular 

 uses and the breeder strives to produce a race with the 

 most suitable composition for each. The following para- 

 graphs will furnish concrete examples of how the breeder 

 has solved some of these problems : 



The Methods of Plant Breeding naturally vary 

 according to the problem, but may be grouped under two 

 general heads, selection and hybridization. The student 

 will understand from his previous studies that some kinds 

 of plants are naturally self -pollinated and so exist in 

 " pure lines " ready for the breeder to identify and select, 

 while others are naturally cross-pollinated so that popula- 

 tions of them consist of many and usually all possible 

 genotypes. Some plants are self-sterile and so cannot be 

 self-pollinated, thereby increasing the labor of breeding 

 them. Furthermore, it frequently happens that it is 

 necessary to cross two species or varieties in order to 

 bring together the desired combination of characters, 

 self-pollinate or interbreed the hybrids, and choose from 

 among the recombinations in F2 and later generations 

 the ones that are desired. 



Pure Line Selection. — Wheat is naturally self -polli- 

 nated but is occasionally crossed in the field. A field of 

 most commercial varieties is likely to consist of a mere 

 mechanical mixture of numerous different pure lines. One 

 method of improving it is, therefore, to make a very care- 

 ful and complete study of the growing grain in order to 

 choose the best plants. These are then harvested sepa- 

 rately. The seeds of each plant will ordinarily be pure 

 and constitute a pure line. These pure seeds are sowed 

 the next year in the testing plot, one row to each collec- 

 tion of seed, for comparison and selection of the best pure 

 lines. The next year samples from the best pure lines 

 are sowed in larger plots for more accurate comparison. 

 It requires about five years of testing in this way to 

 make a final selection. It was in this way that Professor 



