140 SOUTHERN FIELD CROPS 



same plant. This relationship is too close for best yields, 

 especially if the process be continued for several years. 



Close-breeding consists in crosses made among the silks 

 and tassels of plants all of which sprang from grains borne 

 in the next preceding generation on one ear. This rela- 

 tionship is so close as to incur the danger of reducing the 

 yield of grain. 



Cross-breeding consists in crosses made between plants 

 that are not related. This may be 



(a) Between unrelated plants of the same ^'ariety ; or 



(6) Between different varieties of the same race, as 

 yellow and white dent corns ; or 



(c) Between different races, as sweet and dent corn. 

 As a rule, the most desirable relationship is cross-breeding 

 between unrelated plants of the same variety. 



128. Effects of in-breeding and of cross-breeding on 

 yield. — Experiments have shown that continued self- 

 fertilization of the corn plant reduces the yield ; and when 

 self-fertilization is practiced for several successive genera- 

 tions, it may dwarf the stalk and finally result in some 

 measure of sterility (Figs. 75, 76). Halsted (N. J. Expr. 

 Sta., Bui. No. 170) found that self-pollination in sweet 

 corn tended to increase the percentage of alliino plants; 

 that is, those with white foliage — an undesiral)le quality. 



Cross-breeding, on the other hantl, invigorates the strain, 

 and some recent experiments show that it may greatly 

 increase the yield in the first generation of cross-bred 

 plants. But it should be rememl:iered that cross-breeding 

 of dissimilar types has the serious disadvantage of destroy- 

 ing uniformity. It should be confined chiefly to plants 

 of the same variety, or to very closelv related varieties. 



