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, 7.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 F, 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 
Digitized by Microsoft® 
