506 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
husks, the more completely the ear is enclosed and the less capable 
it is of dispersing its seeds. 
In short, a rather simple change but a very important one, the 
lowering of the position of the ear (comparable, perhaps, to moving 
the engine of a primitive airplane from a position behind the wings 
to one in front of them), has separated the sexes, and made for a 
larger strictly grain-bearing ear which is completely protected by 
the husks and is no longer capable of dispersing its seeds. In short, 
a mutation at a single locus on chromosome 4 has made the corn plant 
less able to survive in nature but much more useful to man. 
The last two plants in figure 4 show some of the changes which 
human selection has subsequently effected. Selection for large ears 
has tended to eliminate the secondary stalks and to reduce the number 
of ears per stalk. The fifth plant in figure 4 represents a typical New 
England flint corn in which the secondary stalks have been reduced 
to low tillers, known to the farmer as “suckers,” which in days of 
cheaper labor were often removed under the erroneous impression 
that their removal was a kind of beneficial pruning operation. The 
last plant represents a typical Cornbelt dent corn which is predomi- 
nantly single-stalked and often bears only one ear, in approximately 
the middle region of the stalk. 
The corn plant has a distinct advantage over other cereals in bear- 
ing its ears in the middle region of the stalk, which, being thicker 
and stronger than the tip, is capable of supporting a larger ear. 
This is a simple and obvious mechanical advantage. There may also 
be a less obvious but even more important physiological advantage. 
We have evidence‘ that, under otherwise constant conditions with 
respect to the genotype and the environment, a decrease in the weight 
of the tassels may be accompanied by an increase five times as great 
in the weight of the ears. There is at least little doubt that corn, 
by virtue of its botanical characteristics, is potentially more produc- 
tive than the other cereals. For example, record yields of wheat sel- 
dom exceed 100 bushels per acre; the maximum yields of corn recently 
reported are more than 300 bushels per acre. 
There have, of course, been other factors, not discussed here, in 
corn’s evolution under domestication: mutations at many loci in addi- 
tion to that governing the characteristics of pod corn; extensive 
hybridization among distinct races [9, 10]; repeated hybridization 
with teosinte [8, 11] and perhaps also with Z7’ripsacum [6]; and 
human selection for many different characteristics. But it was this 
one mutation at the pod-corn locus—this single change in a molecule 
of the hereditary material—which more than any other factor has 
determined the botanical characteristics of modern corn and which 
«A report on this evidence is in preparation. 
