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 ealled 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. Cressiiy is another term used in- 
stead of hybridization, and the result of the process or hybrid 
is called a cross. 
Fig. 160. Results of hybridizing plums 
-A,astoneless wild plum; B, (, D, E, fruits of seedlings obtained by crossing 
-{ with the French prune. .\bout 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 Wendel’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. 
