136 EDGAR ANDERSON AND WILLIAM L BROWN 



its specialized leaves or husks. When dissected out, these ear shoots (or 

 shanks) are diagnostically different in Northern Flints and Southern Dents 

 (Fig. 8.4). In the former they are long, with elongated internodes which are 

 widest between the nodes, and which have a smooth surface upon drying. In 

 the latter they are very short, frequently wider at the nodes than between 

 them, and have a characteristically ribbed surface upon drying. 



The leaves of the Northern Flints are long and slender and frequently a 

 light green. Those of the Southern Dents are proportionately wider and 

 shorter and are often dark green. They are set upon culms whose internodes 

 are proportionately longer and more slender in the Northern Flints, and less 

 prone to become greatly shortened at the internodes immediately above the 

 ear. 



If we ignore such abnormalities as differences in carbohydrate composition 

 and condensation, these two races of Zea mays still are widely different from 

 one another — as compared to differences between their wild relatives in the 

 Andropogoneae or the Maydeae. The differences in internode pattern and 

 proportion and in leaf shape are similar to those frequently found between 

 species of the same genus. The differences in pedicellation of the upper spike- 

 let would be more characteristic of genera and sub-genera. On the other 

 hand, in the whorling of the central spike (whorls of 2 versus whorls of 3) 

 is the kind of difference which would ordinarily separate genera or even 

 groups of genera. On a par with this difference are those in the cupule (the 

 bony cup in which the kernels are attached in pairs). They are so difficult to 

 observe that we cannot discuss these until the general morphology of this 

 organ has been described. If we sum up the morphological evidence, it is clear 

 that the fundamental differences between the Northern Flints and the 

 Southern Dents are similar to those which differentiate distantly related 

 species (or even genera) among related wild grasses. There is every morpho- 

 logical indication, therefore, that we are dealing with two fundamentally 

 different germ plasms. 



The cytological facts reported above lend further weight to the conclusion 

 that the Northern Flints are basically different from the Southern Dents. 

 The former have chromosomes which are essentially knobless at pachytene. 

 The latter average nearly one knob per chromosome (Fig. 8.2). Heterochro- 

 matic knobs are known in other grasses besides Zea mays. In these other 

 genera, their presence or absence, from such evidence as is available, seems 

 to be characteristic of whole species or groups of species. Such a difference 

 between the Flints and Dents indicates that we are dealing with two funda- 

 mentally different germ plasms. It has been shown in Guatemala (Mangels- 

 dorf and Cameron, 1942) and in Mexico (Anderson, 1946) that the varieties 

 with many knobs are morphologically and ecologically different from those 

 with low numbers of knobs. 



A further indication that these two germ plasms are physiologically dif- 



