a few of the samples were free of tripsacoid cobs. 
Two of the fields where tripsacoid cobs had been har- 
vested were visited and teosinte was found to be abun- 
dant around the field margin. Although most of the 
teosinte plants had dispersed their seed and were dried 
and broken, several Fy hybrids were located which still 
possessed intact cobs. In one field, two teosinte backcross 
progeny of the F; maize Xteosinte hybrids were discov- 
ered, indicating that there issome reciprocal introgression. 
Despite the presence of reciprocal introgression, most 
of the gene flow is from teosinte to maize. Teosinte is 
a wild plant possessing the ability to disperse its seed as 
single rachis segments, while domesticated maize has a 
massive cob tightly enclosed by a husk system. Only 
those teosinte backcross segregates which possess the 
genetic control fora disarticulating rachis are able to dis- 
perse their seed. This factor is primarily responsible for 
the unidirectional flow of genetic material. The back- 
crosses to teosinte that segregate a more maize-like cob 
with non-disarticulating rachis disperse the entire spike 
as a whole. The numerous seed all germinating still at- 
tached to the cob are so crowded that they either choke 
each other out or develop into numerous spindly plants 
that fail to flower. In both cases, the genetic inheritance 
of the maize-like cob is lethal to a plant dependent on 
natural dispersal of its seed for survival. The selection 
tor the disarticulating teosinte pistillate spike and dis- 
tribution of single seeds protected by a rachis-segment, 
along with a large population of wild plants in the sur- 
rounding region, appears to act against the effects of 
maize introgression on the pistillate spike of teosinte. 
Study of maize ears 
The evidence for introgression of tripsacoid characters 
from teosinte is objectively measurable in the morpholo- 
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