PLANT BREEDING 



187 



of no use in fertilizing, but sometimes it succeeds perfectly, 1 

 as when pollen from one species of plum is used for pollinating 

 flowers of another species, or when one parent flower is a plum 

 and the other a cherry, or one a plum and the other an apri- 

 cot. Any plant grown from seed thus produced is called a 

 hybrid. The terms hybrid and hybridization are also coming 

 to be generally used in cases of breeding between varieties as 

 well as between species. Crossing is another term used in- 

 stead of hybridization, and the result of the process or hybrid 

 is called a cross. 



ABC r> 



FIG. 160. Results of hybridizing plums 



A, a stoneless wild plum ; B, C, D, E, fruits of seedlings obtained by crossing 

 A with the French prune. About one half natural size. Modified from a photo- 

 graph by Burbank 



Hybrids are often extremely variable (figs. 160 and 161), 

 and for this reason it has become a common practice to hybrid- 

 ize plants for the sake of getting a variety of new combina- 

 tions of characters in the hybrid seedlings, and then to select 

 the desirable kinds for breeding purposes. Until recently it 

 was supposed to be impossible to predict the way in which the 

 characters of the parents would be inherited by the successive 

 generations of hybrids. But a law known from its discoverer 

 as Mendel's law often enables the breeder to foretell the char- 

 acters which the hybrid plants will inherit. Gregor Mendel, 



1 The plants grown from seed which was the result of pollination between 

 different species are often vigorous but incapable of producing seed. This 

 sterility of plants bred by cross-pollination between different species is so 

 common that it was formerly often used as a test to determine whether two 

 kinds of plants that seemed to be different species were really such or were 

 only varieties. 



