350 



EVOLUTION AND GENETICS 



discard the fifty per cent of homozygous individuals in every 

 generation. 



Asexual Propagation. Repeated hybridization is naturally more 

 complicated and more expensive than the normal course of re- 

 production. It is avoided by commercial plant breeders through 

 asexual propagation of their hybrid stocks. Hybrid fruits are 

 propagated by grafting scions of the desirable stock onto hardy 

 root systems, sometimes of entirely different species. The beautiful 

 varieties of French lilacs are grafted onto roots of the common 

 lilac or privet. Chrysanthemums are easily raised from cuttings, 

 roses from cuttings or by grafting, peonies and other flowers by 

 division of the roots and crown of the plant, and bulbs through 

 their natural asexual increase. Plants which can be produced 

 only from seed are obviously subject to the same limitations as 

 animals. 



Hybridization for the production of new combinations of char- 

 acters is limited only by the difficulty of isolating homozygous 

 strains. If the desired type is complex this difficulty is great and 

 it is necessary to resort to asexual propagation if possible. Many 

 desirable hybrids are simple, however, so this does not limit the 

 uses of the process to organisms which can be produced asexually. 



Perhaps no useful hybrid is a better illustration of complexity 

 and the value of asexual propagation than Burbank's Alhambra 

 plum. The ancestry of this variety is incorporated in the following 

 diagram by Babcock and Clausen: 



Pissardi 



Kelsey 



What an impossible task it would be to fix the desirable combina- 

 tion of characters in any other way! 



