INHERITANCE IN THE BARLEY SPIKE. 21 
density. More extensive study would be needed to determine whether 
these apparent centers are of any significance. 
DISCUSSION OF RESULTS. 
From the fact that segregates homozygous for density are apparent 
hi the measurements of the F 3 and F 4 generations, it seems safe to 
conclude that internode length in the barley rachis may be explained 
on the factor hypothesis. The number or value of the factors involved 
is not readily estimated. In a general way the results of the Man- 
churia X Svanhals and the Pyramidatum x Jet crosses seem to 
indicate a single main factor difference. The proportion of homo- 
zygotes is roughly satisfactory, and the absence of homozygotes differ- 
ing greatly from the mean of their parents is also in favor of this 
belief. The dominance of density in the F x generation in the first 
cross and its intermediate expression hi the second is of interest. 
The results in the Hanna X Reid Triumph cross in the same way 
indicate a broad difference of two factors. In this cross forms were 
isolated that were homozygous for intermediate densities, as well as 
forms having densities near those of the parents. These results can 
be interpreted very satisfactorily on the basis of two main factors 
for internode length. These factors are cumulative in effect, both 
being necessary to produce the extreme type. The results show that 
a sort may be homozygous for one of the factors and heterozygous 
for the other. At least, heterozygous forms whose progeny range is 
from the intermediate group to one or the other parent are so 
interpreted. 
The Hanna X Zeocriton cross gave homozygous intermediates of 
unlike value, as well as homozygous sorts which were like the parents. 
If the presence and absence hypothesis is here used, three mahi 
factors may be postulated to explain the genetic facts. These factors 
ma} r be supposed to be of like value, each inherited independently, 
each allelomorphic to its absence, the number showing a hetero- 
zygous condition being half the homozygous sorts. This hypothesis 
explains the genetic fact fairly well. Other minor factor differences 
are doubtless necessary to explain all of the results. One known 
minor character of some density significance separates the parental 
forms. This is a difference hi the progressive density from the base 
to the tip of the rach's, the Zeocriton parent being the only sort 
which shows a constant increase in length of internode from the base 
to the tip of the spike. 
A comparison of the Pyramidatum X Jet cross with the Hanna X 
Zeocriton cross illustrates some facts regarding the mode of inherit- 
ance of density. These are the two widest crosses made hi the study. 
The first produced no homozygous intermediates. The second pro- 
duced many. An F x generation was grown of the P3a'amidatum X 
