SOME PRINCIPLES OF PLANT BREEDING 495 



I. THE IMPROVEMENT OF EXISTING VARIETIES. 



665. Cultivated races of plants. Until recent years plant 

 breeders have given their attention chiefly to the production of 

 varieties which differed from each other in regard to form, color, 

 productivity, etc. In recent years increasing attention has been 

 given to improving existing varieties in the direction of increas- 

 ing the percentage of certain constituent substances in the com- 

 mercial plant product. For example, increase in the percentage 

 of sugar in the sugar beet and sugar cane; increase in the per- 

 centage of protein in corn and wheat, of oil in corn, etc. Since 

 these cannot well be distinguished as varieties, there is a ten- 

 dency to apply the term race to highly developed and selected 

 variations of this kind. There are great variations in the per- 

 centages of these constituent substances in plant products. By 

 selecting the seed (or cuttings, grafts, etc., from those propa- 

 gated by asexual methods) from individuals of a good variety 

 which yield the highest percentage in the desired constituent (of 

 course having regard to the proper physical conformation of the 

 plant), planting this and selecting again from the individuals 

 among the offspring of the selected ones, and repeating this for 

 several years, a high standard of excellence can be attained. 

 With these races the process of selection and good cultivation 

 must continue year after year, otherwise deterioration results. 

 The percentage of the desired constituent is determined either by 

 chemical or physical analysis. 



Seed corn (Indian corn or maize) can be selected by a physi- 

 cal analysis. The proportion of starch in the grain is inverse to 

 the proportion of gluten or the size of the embryo. A section of 

 the grain (paragraph 17 and fig. 17) will show to the eye the 

 proportion of these constituents to each other. When the white 

 starchy portion in the center is limited, the hard outer gluten- 

 bearing portion will be greater. If one is selecting for protein 

 content, grains are chosen which have a small starch content, 

 since the horny layer containing gluten (a protein substance) and 

 the embryo (containing protein) are both inverse in bulk to that 



