HYBRIDIZATION 351 



failures indicate that this may be the difficulty it will be worth while to 

 try the application of a film of water or weak sugar solution to the surface 

 of the stigma before pollination. By the aid of this simple device crosses 

 have been secured between certain species of beans which had been 

 repeatedly attempted without success. In this connection it may be well 

 to give a word of warning. While it is always advisable to ascertain 

 what one's predecessors have accomplished or failed to accomplish, the 

 hybridizer should remember that both plants and local conditions are 

 variable, and what may have been impossible at one place may be possible 

 at another. Or the adoption of simple devices such as the water film 

 on the stigma may be the determining factor. Much perseverance is 

 sometimes necessary. 



(c) Susceptibility to Mutilation. Some plants are much more sensitive 

 to mutilation than alfalfa. It appears that some are suceptible to merely 

 removing the anthers from the ends of the filaments. In such cases it is 

 necessary to resort to special methods for protecting the stigma from 

 self-pollination. The details will depend upon the structure of the flower 

 and whether it is protandrous or protogynous. 



Conditions favorable for hybridization may be summarized as follows : 

 ideal conditions for flowering and fruiting; receptive stigmas; viable 

 pollen ; morphological and physiological compatibility between pollen and 

 pistil; resistance of flowers to manipulations. 



Species hybridization is generally more apt to be attended by diffi- 

 culties than is the crossing of varieties, although certain varieties of the 

 same species have been found mutually incompatible in crossing. In 

 general crosses are most successful when made between closely related 

 species. The reason for this is clear when the genotypic differences 

 between distinct species are considered as differences between homol- 

 ogous factors, i.e., factors which condition similar characters as was 

 explained in Chapter XII. It is possible that in very closely related 

 species the factors conditioning similar morphological and physiological 

 characters are themselves similar, if not in a specific sense at least in 

 terms of the whole reaction system. The new combinations of these 

 similar systems of factors which would be formed in FI hybrids, would be 

 compatible with the vital functioning of the zygote including the produc- 

 tion of viable gametes. In widely separated forms, on the other hand, 

 the reaction systems must be very different, thus causing corresponding 

 reduction in the chances of favorable combinations among the hybrid 

 zygotes. While it is impossible to judge with certainty of the possibilities 

 of species crosses by somatic resemblances and differences, yet the 

 taxonomic relationships of forms it is proposed to hybridize serve as a 

 general guide in forming such estimates. No hybrids between different 

 plant families are known and few authentic cases of intergeneric crosses 



