CHAPTER III 



THE MODE OF REPRODUCTION IN RELATION TO 

 BREEDING 



General recognition of the stability of inherited factors has 

 served to emphasize the importance of a knowledge of the mode 

 of reproduction of crop plants. If the crop in question is nor- 

 mally self-fertilized, and has been bred carefully, accidental 

 crosses may cause serious mixtures in the variety and thus 

 prohibit its sale as pedigreed seed. With naturally cross- 

 fertilized plants, self-fertilization often has a detrimental effect. 

 A knowledge of the mode of pollination of a crop is therefore 

 an absolute necessity in outlining correct methods of breeding. 



As with other characters, environmental conditions play an 

 important role. With crops which are adapted for insect 

 pollination and yet which are self-fertile, the number and sort 

 of insects found in the locality may greatly modify the amount of 

 crossing which takes place. Variations in moisture conditions 

 may determine the amount of cross-pollination. The age of the 

 plant also is of importance. Aside from these there are often 

 varietal differences in closely related forms. 



Plants may be placed in four groups according to their mode 

 of reproduction. These groups, however, overlap because of 

 prevailing conditions and inherent differences which the plants 

 exhibit. 



Group 1. Naturally self-pollinated: Wheat, oats, barley, 

 peas, beans, flax, tobacco, tomatoes, cotton, sorghums. 1 



Group 2. Often cross-pollinated: Maize, rye, sugar beets, 

 root crops, grasses, alfalfa, cucurbits. 



Group 3. Cross-pollination obligatory: (a) Self -sterile, red 

 clover, sunflower, many fruits; (b) Dioecious plants, hops, 

 hemp, asparagus, and date palm. 



1 Some crops, such as sorghum and cotton, cross in the field frequently. 

 As there is no sharp line of demarkation between cross- and self -pollinated 

 plants and as sorghum and cotton should apparently be handled by the 

 breeder in much the same manner as crops like barley, which is seldom 

 naturally cross-pollinated, it seemed wiser to place sorghum and cotton in 

 the self-fertilized group. 



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