84 The Third and Fourth Generation 



box and were to shake out twenty-four, there 

 is Uttle Hkelihood that in a hfetime he would 

 twice shake out the very same twenty-four. 



The plant breeder recognizes this fact and 

 propagates his stock by cuttings rather than 

 by the sexual process wherever possible. If a 

 tree is grown from an apple seed the fruit 

 produced is seldom as good as that on the 

 original tree, because when the chromosomes 

 recombine in the sexual process the chances are 

 that so efficient a combination will not again 

 arise. So the grower takes a twig from his tree 

 and plants it so as to get a new tree whose cells 

 possess the same chromosome content as the 

 original. It is only in the sexual process that 

 reduction and recombination of chromosomes 

 occur, and these are the basis of hereditary 

 characters. When, however, he is after new 

 things, better types of fruits or grains, then the 

 breeder not only makes the plants reproduce 

 sexually, but he uses as parent forms plants that 

 have unlike but desirable characteristics, in the 

 hope that some new combination may result 

 that will be better than either of the parents. 



