PLANT BREEDING AND EVOLUTION 



193 



Tassel 



corn are due to natural crossing in the field, since corn is nor- 

 mally cross-pollinating. There is evidence also that every field 

 of corn of the same kind or va- 

 riety is made up of minor races 

 (elementary species), which differ 

 among themselves in some or 

 all of the characters mentioned 

 above. These races cross and 

 produce the same kind of varia- 

 tions as arise from artificial cross- 

 ing and hybridizing. In other 

 instances the variations are due 

 to differences in soil, water sup- 

 ply, climate, and tillage. Out of 

 these multitudinous differences 

 the corn breeder selects the plant 

 or plants which suit his ideal, 

 and proceeds to secure an im- 

 proved race by careful selection 

 and cultivation. Fig. 103 illus- 

 trates the great variability in ears 

 of corn in the same field, due to 

 the indiscriminate crossings be- 

 tween plants bearing poor and 

 good ears. Fig. 104 illustrates 

 the great improvement in uni- 

 formity and yield which can be 

 secured by selecting the best ears 

 for seed and by preventing cross- 

 pollination with inferior types for 

 a series of years. 



The gradual increase in yield 

 of corn in Illinois, Iowa, and 

 other great corn-producing states 

 during the past decade is due in large measure to the fact that 

 corn breeders have exercised more care than they formerly did 

 in studying variations in corn and in selecting proper seed for 



Secondary 



roots Primary root 



FIG. 102. A plant of Indian corn 



Showing male inflorescence (tassel) and 

 ears ready for pollination 



