268 



BREEDING CROP PLANTS 



condition of most fruit material. A commercial variety may be 

 extremely valuable and yet be heterozygous for many characters. 

 On the other hand, the commercial variety may be homozygous 

 for a large part of its characters. It seems reasonable to con- 

 clude that the more nearly homozygous the parental variety, 

 other things being equal, the greater value it would have as a 

 parent. 



The ability of impressing its characteristics upon the larger 

 part of its offspring has been called prepotency by animal breeders. 

 Such prepotency is genetically explained by the supposition that 

 the prepotent parent is homozygous for certain dominant factors 

 for the characters under observation. Hedrick and Wellington 

 (1912) showed that some crosses between apple varieties pro- 

 duced a considerable percentage of individuals with small fruits. 

 Thus the cross between Rails and Northern Spy gave great 

 variability in size of apples, while the cross between Sutton and 

 Northern Spy gave progeny in which no trees were obtained 

 which produced small fruit. One of the great difficulties is that 

 it takes several years to learn the varieties which when crossed 

 will give certain desired combination. 



Another difficulty which must be considered is that many 

 varieties of fruits are self-sterile. This is of utmost importance 

 in commercial fruit production for it is necessary to interplant 

 such a variety with some variety which produces an abundance of 

 pollen which is capable of fertilizing the variety in question and 



TABLE LXXI. INCREASE IN WEIGHT OF SEED AND FRUIT DUE TO CROSS- 

 POLLINATION 



