ORIGIN AND SIGNIFICANCE OF CORN BELT MAIZE 141 



made it an outstandin<f source of inbreds of proven highly sj)ecific combining 

 ability when used with other Corn Belt inbreds. This is not an isolated ex- 

 ample, and even more extreme cases could be cited. We think it is a reason- 

 able working hypothesis that Northern Flint varieties of superior productiv- 

 ity might be efficient sources of improved heterozygosity for the United 

 States Corn Belt. 



Morphological Characters as Related to Heterosis 



To put this hypothesis in different language, morphological characters, if 

 carefully chosen, may be used as criteria of specific combining ability in Corn 

 Belt inbreds. Before presenting data bearing directly on this hypothesis, two 

 points need to be emphasized and discussed: (1) the effective selection of 

 morphological criteria, and (2) the relativity of all measures of effective 

 combining ability. 



Previous studies (Kiesselbach, 1922; Jenkins, 1929; and others) have indi- 

 cated that the only positive correlations between the morphology of inbreds 

 and their combining ability are those involving characters of the inbreds 

 which are indicative of plant vigor. Reference to these investigations shows 

 that the characters chosen were such superficial measurements as date of 

 silking and tasseling, plant height, number of nodes, number of ears, ear 

 diameter, etc. Unfortunately, the morphology of the maize plant is not a 

 simple matter. It is so complex that one needs technical help on morphology 

 quite as much as he would in biochemistry were he studying the concentra- 

 tions of amino acids in the developing kernel. 



Accordingly, we first familiarized ourselves thoroughly with the technical 

 agrostological facts concerning the detailed gross morphology of grasses in 

 general and Zea in particular. Just as in the case of a biochemical study of the 

 kernel, we found that further original research was necessary if the investiga- 

 tion was to be carried on effectively. We have accordingly undertaken de- 

 tailed studies of internode patterns and branching of the inflorescence; the 

 venation, size, and shape of the male spikelet, the development of the husk 

 leaf blades, the external anatomy of the cob, and the morphology of the 

 shank. Some of these investigations are still continuing, and must continue 

 if inbred morphology and combining ability are to be effectively correlated. 



It is impossible to produce an absolute measure of combining ability. 

 When one speaks of combining ability of two inbreds, he always refers to 

 their behavior with each other compared to their behavior with certain other 

 inbreds or open-pollinated varieties. This is such a relative measure that the 

 scoring of a particular Fi cross as very low or very high in combining ability 

 might depend solely upon our previous experience with the two inbreds. We 

 may illustrate this point with an extreme example. Let us suppose that we 

 have inbreds IF and 2F derived directly from Northern Flints, and inbreds 

 lOD and IID derived from Southern Dents. Were we to cross IF X 2F and 



