Sterility of the clone of T77psacum laxum is undoubt- 
edly due to irregular chromosome behavior at meiosis. 
Most of the resulting microspores receive more than the 
regular haploid number of chromosomes. 
There is no evidence that 7'ripsacum australe was in- 
volved in the hybridization of the aforementioned triploid 
form of T’ripsacum lawum; no marked chromosomes of 
the former were identified in the sporocytes of the latter. 
The form of diploid 7. australe investigated is apparently 
different, however, in chromosome constituents from the 
one previously reported on by Graner and Addison 
(1944), since they did not find any knobs in their material. 
CONCLUSIONS AND SUMMARY 
The tropical form of T’ripsacum laxum from Colombia 
is triploid with 54 chromosomes. The meiotic chromo- 
some behavior was extremely irregular, and this phenom- 
enon may serve as an explanation of the sterility of the 
plants. It is suggested that this tropical form of T'rip- 
sacum originated by interspecific hybridization between 
a tetraploid form of 7. /avum and an unknown diploid. 
A second tropical form of Tripsacum from Colombia, 
T. australe, is a diploid with 18 bivalents in the micro- 
sporocytes and 86 chromosomes in the somatic cells. 
Meiotic behavior of the chromosomes was regular. It 
apparently was not involved in the course of evolution 
of the triploid T'ripsacum lawum. 
Internal knobs which varied in size were observed in 
both of these tropical forms of T'’ripsacum. Also, in gen- 
eral, the short arms of the nine short chromosomes in 
both forms were more heteropycnotic than in those of 
maize. A diagram (Plate X XI) of the 18 chromosomes 
in Tripsacum australe is appended. The arm ratio and 
length of each chromosome are given. 
[ 106 ] 
