336 WEATHERWAX: THE EVOLUTION OF MAIZE 
pistillate. In the pistillate spikes, or portions of spikes, the 
pedicelled spikelet and the lower flower of the sessile one became 
aborted (Fics. 27, 28). Each branch bearing a female inflores- 
cence shortened enough that the inflorescence was covered by the 
sheath of the leaf just below it; at the same time, all but one (or 
rarely more) of the spikes of this inflorescence disappeared, and the 
two stigmas of each pistil, lying side by side and elongating to 
keep their tips exposed beyond the enveloping leaf sheath, united, 
except at their exposed tips, to form the silk. The brittle rachis 
came with the hardening of the outer glume of the spikelet and 
the walls of the alveolus in which the spikelet was imbedded. 
The leaf whose sheath enclosed the female spike has since then 
almost lost its lamina (Fic. 25). By a continued shortening of 
the internodes, the sheaths of lower and larger leaves were later 
caused to envelop clusters of these simple female spikes and their 
husks (Fic. 26). There is nothing to indicate when Euchlaena 
became an annual, but this habit i is probably a acerag with a 
reduced supply of moisture. 
Origin of Tripsacum.—In those individuals a toward 
Tripsacum, monoecism came about as a result of the loss of female 
elements from the upper part and male elements from the lower 
part of each spike of the inflorescence (Fic. 32). The point of 
‘division between the male and female portions of a spike was 
higher in the inflorescences of the branches than in that of the 
main stem. In the male portion, the pedicellate spikelet became 
almost sessile; the female portion became essentially as in teosinte. 
There was no appreciable shortening of any internodes, and the 
inflorescence was not covered by a leaf sheath; consequently 
there was no reduction in the number of its branches, and the 
two stigmas have persisted (Fic. 31). This plant usually grows 
in rich, wet soil, and it has remained perennial; its ability to 
produce viable seeds is restricted. 
Origin of Zea.—In the evolution of Zea from its progenitor, 
the inflorescence terminating the main stem lost its female ele- 
ments and approached its present form (Fic. 2). Those terminat- 
ing some of the primary branches near the middle of the main 
stem lost their male elements, and these branches began to shorten 
their internodes. As this shortening continued, the secondary 
