half-tunicate (tuk), weak-tunicate (tww), and non-tunicate 
(tuw)—at this locus has already been demonstrated (Man- 
gelsdorf, 1948). In addition, a condition intermediate 
between weak-tunicate and half-tunicate is found in some 
races of maize including the Chapalote maize of Mexico. 
Since this form of tunicate, which has not yet been 
studied genetically, produces approximately the same 
effect when homozygous as does half-tunicate when het- 
erozy gous, it may appropriately be called fourth-tunicate 
and tentatively given the symbols tw/, subject to verifi- 
‘ation by later genetic tests. 
The importance of these 7'w alleles in connection with 
our present studies lies in the fact that a change from a 
higher to a lower allele in the series is almost invariably 
accompanied by a drastic change in the cob/rachis ratio. 
In other words, when the glumes become shorter, the 
rachis, other factors remaining constant, automatically 
becomes thicker. This is easily demonstrated in modern 
isogenic stocks which are segregating for two alleles in 
the Ju series. A comparison of three such stocks is 
shown below: 
Stock Cob/Rachis Index 
Inbred A158 tuhtuh 2.9 
_ “*  tuhtu 2.1 
ve “tutu 1.5 
The cob/rachis index, therefore, is in substantial part 
a simple estimate of the degree to which a variety of 
maize has departed from a primitive tunicate form to- 
ward the modern non-tunicate condition. It must be 
realized, however, that in addition to changes in both 
the diameter of the rachis and that of the entire cob re- 
sulting from changes in 7'w alleles, there are other differ- 
ences resulting from the action of a vast number of 
modifying factors. 
[ 220 | 
