of Tripsacum germplasm. This becomes especially im- 
portant in considering the correlative hypothesis of Man- 
gelsdorf and Reeves (13) that new types of maize orig- 
inating directly or indirectly from the hybridization of 
maize and Tripsacum comprise the majority of the vari- 
eties of Central and North America. 
Teosinte (Tripsacum) germplasm consistently has cer- 
tain definite effects upon the structure of the ear of maize. 
These effects vary considerably with the amount of teo- 
sinte germplasm involved and with the genetic level at 
which it is operating. In spikes approximately interme- 
diate between those of maize or teosinte (spikes similar 
to those borne on F plants of maize-teosinte hybrids), 
small differences in the proportion of maize and teosinte 
germplasm can mean drastic changes in external mor- 
phology, for example: from distichy to polystichy or 
from single spikelets to paired spikelets. As the segre- 
gates approach either end of the range, however, much 
larger increments are necessary to produce these changes. 
These facts, revealed by Fe segregates of maize-teosinte 
hybrids, are even better illustrated by stocks in which an 
entire teosinte chromosome or a part of a chromosome 
has been transferred to a uniform inbred strain of maize 
by repeated backcrossing accompanied by selection. 
Stocks developed in this way are isogenic in nine of their 
ten chromosomes and therefore approximately identical 
with the original inbred strain of maize. The remaining 
chromosome, however, has been substituted for, wholly 
or in part, by a corresponding chromosome from teosinte. 
The substitution of a small amount of teosinte germ- 
plasm for maize germplasm at this level has no apparent 
effect whatever upon such characteristics as paired and 
single spikelets; the spikelets remain completely paired. 
But these doses of teosinte germplasm do consistently 
reduce the number of rows of grain and they consistently 
[ 58 | 
