ORIGIN AND SIGNIFICANCE OF CORN BELT MAIZE 137 



ferent is given by their pachytene behavior. The pachytene chromosomes of 

 the Northern Flints are easy to smear and give sharp fixation images. South- 

 ern Dents are more ditlficult to smear. The chromosomes do not spread out 

 well and do not stain sharply. This is not a result of differences in knob num- 

 ber, since some of the Mexican Dents with few knobs are equally difilicult to 

 smear. Whatever the physiological significance of this reaction, it is direct 

 evidence for a difference in the chemistry of the germ cells. Again such dif- 

 ferences in stainability are more often met with, between genera, than they 

 are in different strains of the same species. 



There is genetic evidence for the difference between Southern Dents and 

 Northern Flints, in the behavior of crosses between them. The F/s are fully 



TABLE 8.1 



PERCENTAGE OF STERILE OR BARREN PLANTS IN 



GOURDSEED, LONGFELLOW, AND Fa GENERATION 



OF CROSS GOURDSEED X LONGFELLOW 



Total 

 Number 

 of Plants 



Gourdseed 



Longfellow 



Fo Gourdseed X Longfellow 



fertile and exhibit extreme hybrid vigor. The Fo's show a high percentage of 

 completely barren plants — plants which formed ears but set little or no seeds, 

 either because of sterility or because they were too weak to mature success- 

 fully — and plants which managed to set seeds, though their growth habit 

 indicates fundamental disharmonies of development. 



Table 8.1 shows the percentages of good ears and plants which were either 

 without ears or on which the ears had failed to set any seed, for Gourdseed- 

 Dent, Longfellow Flint, and their F2, when grown in Iowa. Like Southern 

 Dents generally, the Gourdseed is less adapted to central Iowa than is Long- 

 fellow Flint. An F2 between these two varieties, however, has a much greater 

 percentage than either parent of plants which are so ill-adapted that they 

 either produce no visible ear, or set no seed if an ear is produced. Similar 

 results were obtained in other crosses between Northern Flints and Southern 

 Dents, both in Missouri and in Iowa. From this we conclude that they are 

 so genetically different from one another that a high percentage of their F2 

 recombinations are not able to produce seed, even when the plants are care- 

 fully grown and given individual attention. 



SUMMARY 

 The common dent corns of the United States Corn Belt were created 

 de novo by American farmers and plant breeders during the nineteenth cen- 



