932 
Journal of Agricultural Research 
Vol. XXX, No. 10 
Since the factor for the 6 -rowed 
character is linked with the factor for 
early heading, the 6 -rowed lines and the 
2 -rowed lines should be examined to 
see if a correlation exists for relation to 
Helminthosporium, irrespective of the 
characters early and late. When the 
average Helminthosporium figure for 
the 6 -rowed early lines is compared 
with that for the 2 -rowed early lines, a 
difference is found still to exist for 
Helminthosporium reaction (Table 
XXV). A difference is found also 
between the 6 -rowed lates and the 
2 -rowed lates, likewise between the 
6 -rowed lines and the 2 -rowed lines 
which are segregating for early versus 
late. If each of these groups is given 
equal weight, the figure for the 6 -rowed 
lines is 17.9 ±0.2 6 and for the 2-rowed 
lines 19.6 ±0.4, the difference being 
2 ±0.4, which is significant when 
considered in the light of its probable 
error. 
The differences between the Helmin¬ 
thosporium figures of the early-heading 
and late-heading lines is greater than 
the difference between the figures for 
the 2-rowed and 6 -rowed lines. This 
difference may be due to a physiological 
resistance of lateness or to a more in¬ 
tense linkage between the factor for 
earliness with the factor for suscepti¬ 
bility than exists between the factor for 
6 -rowed and the factor for suscepti¬ 
bility. 
In considering the degree of resist¬ 
ance or susceptibility in relation to 
other characters, one is justified in the 
conclusion that at least three factor 
pairs are concerned in the production 
of resistance and susceptibility. One 
of these factor pairs is located in the 
chromosome pair carrying factors for 
2 -rowed and 6 -rowed, one in the 
chromosome pair carrying factors for 
black and white glumes, and the other 
in the chromosome pair carrying factors 
for rough awn and smooth awn. 
The data given here on the relation 
of factors for resistance to Helmin¬ 
thosporium and the method of locating 
such factors through a study of their 
relation to factors for other characters 
were presented to the Genetics Group 
of the American Association for the Ad¬ 
vancement of Science (2). Sax (20), 
at the same meeting, gave the results 
of studies in which he used a similar 
attack on the location of size factors. 
As Sax (19) points out, the total effect 
of the factor or factor group can not 
be estimated on account of the fact 
that an apparent difference in such 
factors may be due to differences in the 
intensity of their linkage with those 
factors with which they are associated. 
When, however, a sufficient number of 
factors have been located in a particu¬ 
lar chromosome, it may be possible to 
locate a size factor definitely through 
studying its relation to three or more 
other factors. 
SUMMARY 
Each of the cultivated barley species, 
Hordeum vulgare , H. intermedium , H. 
distichon, and H. deficiens , has 14 
somatic chromosomes. According to 
this, 7 linkage groups are expected. 
Each of the following character pairs 
is shown to differ by a single genetic 
factor: 2 -rowed versus 6 -rowed, de¬ 
ficiens versus 6 -rowed, black versus 
white glumes, early heading ver¬ 
sus late, rough awn versus smooth, 
and intermediate-smooth awn versus 
smooth. 
In the cross SvanhalsX Lion the char¬ 
acter pairs black glumes versus white, 
2 -rowed versus 6 -rowed, rough awn 
versus smooth awn, and intermediate- 
smooth awn versus smooth were shown 
to be independent of each other in 
inheritance. Black glumes versus white 
glumes and deficiens versus 6 -rowed 
w T ere found to be independent character 
pairs in the cross H. deficiens steudelii 
X Manchuria. 
Resistance and susceptibility to 
Helminthosporium sativum are shown 
to be due to definite genetic factors 
By studying the reaction of F 3 lines 
to this pathogene in relation to other 
characters the inference is drawn that 
at least three factors are concerned in 
the production of resistance of the 
type possessed by Svanhals. One fac¬ 
tor was linked with the factor for 
2 -row^ed, one with the factor for rough 
awn, and one with the factor for 
white glumes. 
The factor for early heading was 
found to be linked with the factor 
for 6 -rowed. The linkage intensity 
was very low, the crossover value 
being 42 per cent. The linkage of the 
factor for susceptibility to Helmintho¬ 
sporium with the factor for earliness 
is much more intense than that with 
the factor for 6 -rowed, else earliness 
in itself predisposes the plant to attack 
by the pathogene. 
From the results of the crosses dis¬ 
cussed four linkage groups have been 
established. 
6 The probable error of an average of averages calculated according to the formula E— 1/iV 
•• • • Wen 2 , in which n is the number of lines in a group, t the probable error of the 
group, and N the total number of lines considered (17). 
