larly on the ektexine. Dark areas embracing several 
spinules can be seen, no doubt due to phase retardation 
of an incised reticulum, as suggested by Rowley (1960). 
Our sample of a maize- 7’ripsacum hybrid exhibits this 
pattern. 
In a large number of the races of maize, the spinules 
are located very regularly, 1.e., the spacing between each 
spinule being almost equal. With most varieties of teo- 
sinte, the spacing of spinules appears less regular, and in 
some they are rather closely aggregated, appearing as 
clumps. 
The spinules of maize appear to produce more phase 
change than those of teosinte. The spinules appear 
stronger and darker. In teosinte, at the level of the 
spinules, only small areas can be brought into sharp 
focus. In addition, within an area so focussed, there are 
numerous slightly obscured or darkened regions. These 
may be due to one or more of the following factors: 1) 
teosinte may have a faintly incised reticulum: 2) the 
exine of most varieties of teosinte may be thinner and 
more easily deformed (thus thrown out of focus): 8) the 
spinules or columellae of maize may be slightly longer 
than those of teosinte. 
Fossil grass grains were studied in preparation of sedi- 
ment samples from the Bellas Artes core in Mexico City. 
On the basis of the pore-axis ratio, Barghoorn, Wolfe, 
and Clisby (1954) identified certain grains from the lower 
reaches of this core as maize. The precise age of these 
sediments is not known, but it is beyond reasonable 
doubt that they antedate man’s entrance into the New 
World. We re-examined grass pollen grains from the 
Bellas Artes sediments which, on the basis of size meas- 
urements, appeared to be either maize, teosinte, or T'7ip- 
sacum, Then, using the optical criteria described earlier 
in this paper, some of the grains were identified as maize, 
[ 42 | 
