breaking the ear in two. It has already been mentioned 
that the cross section of a shelled cob comprises two con- 
centric circles, the inner enclosing the zone of rachis tis- 
sue, and the outer the zone of glume tissue. The four 
upper diagrams in Plate X XVIII show cross sections 
of four cobs from Bat Cave and illustrate a sequence of 
changes in the cob/rachis index. 
When the kernels are still attached to a cob, the cross 
section comprises three instead of two concentric circles, 
the diameter of the third circle representing the distance 
to the dorsal surface of the kernels. In true pod corn 
(tuhktuk and Tutu in Plate XXVIII) the third circle may 
lie within the second, but in all other types of maize it 
lies without. Now, if the average length of the kernels 
themselves is ascertained (for example, by measuring ten 
kernels laid end to end), the point at which the base of 
the kernels is attached to the rachillae is easily deter- 
mined by measuring in from the third circle a distance 
equivalent to the average length of the kernel. ‘These 
distances are shown by broken lines within the glume 
zone in the diagrams in Plate XXVIII. All of these are 
based upon actual diameters of the three concentric cir- 
cles described above and, although fundamentally simple, 
are highly significant. They reveal, for example, that in 
the three genotypes tuAtuh, tuktu and tutu from a relative- 
ly isogenic stock derived by repeated back-crossing to In- 
bred A158, there is a progressive increase in the diameter 
of the rachis, a progressive decrease in the size of the 
glume zone and a progressive shortening of the distance 
between the surface of the rachis and the base of the ker- 
nels. A similar comparison between J'utu and tuwhktu, also 
in a relatively isogenic stock resulting from back-crossing 
to Inbred P39, illustrates a similar situation. In both of 
these series the changing relationship of rachis, glumes 
and kernels is entirely a matter of alleles at the T'u-tu 
[ 241 ] 
