crease in mean length from stratum to stratum. Even 
more interesting than the mean length of the cob is a 
comparison of the maximum and minimum lengths. 
Beginning with the fourth stratum there is a progres- 
sively greater divergence between the shortest and the 
longest ears in each stratum. Part of the increase in range 
of variation may be attributed to a progressive increase 
in the size of the samples available, but there is no doubt 
that there is a true increase in the range of variation be- 
yond that resulting from sampling. This is reflected not 
only in the size of the cob but in many other characters 
as well. 
Diameter of the Rachis and Cob. When a cob of maize 
is snapped in two it is quite easy, in examining the cross 
section, especially of the lower half, to distinguish be- 
tween an inner zone made up of the rachis and an outer 
zone made up of the glumes. These zones represent two 
concentric circles whose diameter is easily determined 
with a pair of calipers. The data in Table I reveal that 
there are no significant differences in the diameters of the 
rachis and the entire cob in the three lowest strata. 
Thereafter, however, there is a progressive increase in 
mean diameter of the rachis and in mean diameter of the 
entire cob from stratum to stratum. 
The Cob/Rachis Index. Much more impoicant than 
the actual diameter of these two concentric circles rep- 
resenting the rachis and the entire cob is the ratio of one 
to the other. This ratio, called the cob/rachis index and 
obtained by dividing the larger figure by the smaller, 
may prove to be one of the most useful measurements 
yet employed in studying and classifying maize varieties, 
for it has a clear-cut and definite genetic basis involving 
changes in alleles at the 7'u-tw locus on the fourth chro- 
mosome. The existence of four alleles—tunicate ( 7'w), 
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