228 Science of Plant Life 



may show some characters of each. In the second generation 

 derived from crosses some show a wide range of variation, with 

 all possible combinations of the characteristics of the parent 

 plants. Successful hybridization, therefore, frequently in- 

 creases the number of variations available for selection by 

 the plant breeder. 



In many plants hybridizing has a physiological effect 

 which is of importance, in that it increases the vigor of the 

 offspring. In simflowers, for example, the hybrids secured 

 by crossing the American sunflower and the Russian sun- 

 flower, neither of which was over lo feet in height, grew 

 under the same conditions to a height of 15 feet. 



Selection. Variations, mutations, and the results of hy- 

 bridization furnish the material from which valuable new 

 varieties of animals and plants may be selected. The plant 

 breeder selects from among the hundreds or thousands of 

 plants grown in trial grounds those individuals that most 

 nearly approach the form or quality desired. The seeds of 

 these plants are kept separate and are planted the following 

 season. This process of selection may be repeated year after 

 year until a large part of the progeny or all of them show the 

 quality aimed at in the selection. Some variations can be 

 maintained only by continual selection of the best individuals. 

 Others may soon become stable in character, and the plants 

 are then said by plant breeders to breed true. In fruit trees, 

 desirable varieties may be maintained by grafting and 

 budding ; and in other plants that can be propagated vegeta- 

 tively, varieties may be perpetuated by cuttings, bulbs, 

 tubers, and corms. 



Weeds. Another way in which agriculture is affected by 

 reproduction in plants is through the multiplication of un- 



