functioned in direct competition with pure maize pollen 
about 42 per cent of the time. Thus, this particular Trip- 
sacum chromosome is in several respects the counterpart 
of modern maize chromosome 2, although, it is shorter 
in length and may not have the counterpart of the 4 
locus on this particular chromosome in maize. 
Our own data likewise show that the Tripsacum ho- 
meolog of maize chromosome 9 which carries a dominant 
allele of wa also carries dominant alleles of sh; and bz, 
two other recessives whose loci occur on chromosome 9. 
Thus, even the cytogenetic data so far available sug- 
gest strongly that the second genom is much more closely 
related to maize than the X or ‘‘manisuroid’’ genom. 
Accordingly, we are designating this as the “‘maizoid’’ 
or Z genom. This genom may have been derived from 
a wild maize not too different in its characteristics from 
the prehistoric wild maize described by Mangelsdorf et 
al (1964) or it may have come from the remote ancestor 
of that maize. 
Another means for the cytological corroboration for 
the presence of the postulated X and Z genoms within 
Tripsacum is by comparing the frequency of various types 
of synaptic relationships within haploid maize, Z, and 
haploid Tripsacum, XZ, to that of the F; maize-Tripsa- 
cum hybrid, Z (XZ). A model for their expected be- 
havior on the basis of this hypothesis can be set up and 
tested against the observed behavior. Such a model would 
stipulate low intragenomic pairing of the Tripsacum 
chromosomes and considerable intergenomic pairing be- 
tween maize and Tripsacum chromosomes when the 
chromosomes of the two species are brought together in 
the same cell. 
The behavior of chromosome synapsis was studied in 
haploid maize and in hybrids of maize and 7T’ripsacum 
floridanum (a close relative of 7. dactyloides) (Chaganti 
[ 807 ] 
