200 
Journal of Agricultural Research 
Vol. XXVI, No. 5 
roots and the other underground parts or to some other factor is not 
known. Further study is being made to determine this point. 
After each plant in a given series had been given a numerical rating, the 
final infection rating for the plants grown at a given temperature was 
arrived at by adding together all the numerical ratings, dividing this 
sum by the total number of inoculated plants involved multiplied by 
three. This result was then multiplied by ioo, thus putting the infection 
rating on a percentage basis. 
Sum of all numerical ratings X ioo T r 
Total number of inoculated plants X 3"" neC 10n ra 
This result is then the comparative infection rating for the given tem¬ 
perature, since three times the total number of plants (3 being the highest 
numerical rating) represents the highest possibility for disease under the 
conditions of the experiment. The results from all plants grown at all the 
temperatures in a given series are compared on a basis of factors derived 
according to the above method for each separate temperature. 
In cases where some Helminthosporium infection occurred in the con¬ 
trols, the number of such infected plants was deducted proportionally 
from the total number of inoculated plants before determining the disease 
factor in the inoculated series. Usually the uninoculated control plants 
were free from infection, but it was found to be very difiicult to prevent 
all contamination, because of the fact that H. sativum sporulates so freely. 
Results 
Host development. —While the experiments cited were designed pri¬ 
marily to yield data concerning the development of the disease, it has 
been possible also to obtain information concerning the influence of soil 
temperature on the host plant. 
As shown by Dickson (4) and other workers, the host plants react to 
soil temperature in many respects. In the case of the time required for 
the seed to germinate and emerge from the soil, this study shows that 
the higher temperatures, from 24 0 to 34.5 0 C., speed up this process in 
wheat and barley. At 28° emergence takes place in about three days, 
with 32 0 , 34.5 0 , and also 24 0 , coming on in about four days. At soil 
temperatures of 20°, 16 0 , 12 0 , and 8°, emergence takes place at intervals 
of about 5.5, 7.5, 10, and 16 days, respectively, from the date ofplanting. 
Considerable influence of temperature on the development of the 
plants after emergence also was found. During the periods of the ex¬ 
periments it was discovered that the greatest development in stature and 
dry weight of plants took place at temperatures of from 20° to 24 0 C. 
This temperature range forms the rather broad crest of a curve which 
descends gradually toward the higher and lower temperatures. 
At 8° C. germination was slow, but a fairly high percentage of seeds 
germinated. The percentage of germination seemed to be highest at 12 0 , 
16°, and 20°. Owing to the slow development of plants and the slight 
extent of disease at 8° this soil temperature was not used after the 
second experiment with Marquis wheat. It was found that very few 
plants developed at soil temperatures above 35°, and this temperature 
proved impracticable for the disease experiments. Even at 34.5 0 there 
was poor germination and the plants did not thrive. 
It was found that a temperature of 20° C. tends to produce the greatest 
number of tillers in wheat. In 57 days the production of tillers per plant 
