differ. Obviously in crossing two strains with different 

 factors such as: 



AA, BB, CC, DD x aa, bb, cc, dd. 

 the hybrid will be Aa Bb Cc Dd, which is completely 

 heterozygous. If this is "selfed" homozygous conditions 

 are provided with regard to one or more factors with attend- 

 ant loss of vigor. After a time if " selfing' ' be continued, a 

 perfectly homozygous condition is reached. 



Again, the results suggest that the vigor increases with 

 the number of 'desirable' factors present in the hybrid. 

 Jones' theory of dominance of linked characters appears 

 quite plausible as an explanation (See Genetics 2,197). 



Success in hybridization demands technical skill and 

 accurate knowledge as to the .pollination habits of 

 plants. The flower buds are opened carefully and the an- 

 thers removed, before they mature, with a pair of scissors 

 or forceps. They are then enclosed in a loose paper bag to pre- 

 vent the entrance of foreign pollen. When the stigmas are 

 mature ripe pollen is transferred from the anthers of the 

 male parent by means of a brush or by direct contact of 

 anther and stigma, and the paper bag is again fastened over 

 the flowers until fertilization is effected. 



Some plants are normally self-fertilized, while others 

 are normally cross-fertilized. There are many plants, how- 

 ever, that occupy an intermediate position — some self- 

 fertilizing plants being sometimes cross- fertile, and some 

 cross-fertilizing plants sometimes self-fertile. Following is a 

 list of the common economic plants arranged according to 

 pollination habits: 



1. Self-fertilized plants. — 



(a) Normally self- fertilized: — wheat, barley, oats, rice, 



beans, peas, and most legumes. 



(b) Sometimes cross-fertilized :—Tobacco,tomato, flax, 



cotton. 



2. Cross-fertilized plants. — 



(a) Also self-fertilized:— rye, sugar beet, some pears, 



corn and cucurbits. 



(b) Self-sterile:— white and red clovers, alfalfa, sun- 



flower, most apples, some pears, plum, cherry 

 gooseberry, currant. 



(c) Dioecious: — asparagus, hemp, hops. 



Success requires, moreover, the use of large numbers of 

 plants so that an individual with the desired combination 

 of qualities may be secured . Burbank has used this method 



>j'i 



