thickness of this lining is probably an effect of increased 
compression (Plate III, figs. 1, 2, 3). 
The extreme depression of the rachis segments affects 
their vascularization as well. In T'ripsacum, the larger 
bundles adjacent to the cupule lining are twisted into a 
more space-conserving position so that their longest axis 
lies parallel to the surface rather than in the usual per- 
pendicular orientation, as exemplified by both large and 
small bundles from the opposite or convex side of the 
segment. In teosinte, where the effects of compression 
seem even more extreme, there is an actual loss of some 
vascularization. There are fewer strong bundles, and 
these lie just lateral to the dorsal position, so that they 
may serve equally well either of the alternate positions 
of spikelets from successive rachis segments (Plate III). 
This increased compression in teosinte and T'ripsacum 
is likewise apparent in the character of the cells in the 
‘pith’? region. In 7ipsacum, the pith cells tend to be 
flattened in a direction parallel to the surface, whereas, 
in teosinte, the cells are restricted from expansion and, 
as in the glumes, small cells become lignified during 
kernel development (Galinat, 1957). 
The physical effects of pressure in producing the above 
differences may be visualized by an extension of the same 
process used previously to manipulate (hypothetically) 
the pulvinus into resembling the cupule of maize. These 
derivations may be seen in the figures of Plate IV 
as follows: Having depressed the pulvinus (fig. 1) into 
a cupule (fig. 2), as explained previously, further con- 
centrated pressure from single spikelets at the center 
of the cupule would cause the lateral wings to assume a 
position at right angles to that of the cupule and the 
cavity to sink more deeply into the pith, as the general 
structure and anatomical reductions come to resemble 
those of the rachis segments in T'’ripsacum (fig. 6) or in 
[ 25 ] 
