PLANT BREEDING 371 



propagated from accidental hybrids, and a seed of such an 

 apple is for this reason likely to grow into a tree very differ- 

 ent from the parent. Many varieties of potatoes, which 

 are multiplied by cuttings from the tubers, are also hybrids. 

 When a hybrid forms seeds as a result of self-pollination 

 or of pollination between flowers of the same plant, its off- 

 spring commonly " splits up " in accordance with certain 

 fairly definite rules. 1 Some (but by no means all) of these 

 offspring, differently from the original hybrid, breed true 

 that is, their seeds grow into plants like themselves, the 

 seeds of this next generation into similar plants, and so on. 

 It is possible, then, to obtain from some of the offspring of 

 a hybrid new races that can be propagated by means of 

 seeds. Plant breeders may therefore combine hybridization 

 with a selection among the descendants of the hybrid. It is 

 only in quite recent years that the possibility of producing 

 new races in this way has been at all well understood, and 

 even yet we have much to learn about the laws that govern 

 the splitting up of the offspring of hybrids. Still, many 

 valuable varieties have been produced in the past by breeders 

 who selected in a more or less haphazard way the most 

 promising among the descendants of a hybrid that they had 

 obtained either accidentally or intentionally. Now breeders 

 know much better than they did even ten or fifteen years ago 

 what to expect when they produce a hybrid. They may 

 hope to secure from among its offspring a new race that will 

 combine in itself the desirable qualities of both parents of 

 the hybrid. But to secure just the combination of qualities 

 that is wished, it is often necessary to raise the hybrid plants 

 and their offspring in great numbers ; sometimes tens of 

 thousands or even hundreds of thousands of plants must be 

 grown before the exact combination appears. 



1 Discussions of these rules (the "Mendelian laws"), with their modifications and 

 their applications to the production of new varieties, will be found in any modern 

 book on plant or animal breeding. 



