Observations and Experiments on Cereal Rusts. 159 
centage of germination may be observed on account of the age 
of the spores and possibly also on account of the different 
weather conditions to which the material might be exposed 
before its removal from the host. 
From a comparison of the above results with those obtained 
from the controls kept at laboratory temperatures the writer 
has come to the conclusion that in the case of yellow rust there 
is a rapid fall in the germination capacity of the uredospores. 
When kept in the laboratory their viability fell down from 
80 % to 25 % after a week and by the end of a month the spores 
showed only 5 °% germination, whereas the same rust as already 
stated keeps nearly 20 % of its viability at 2°5°C. or 5° C. for 
a month. 
The uredospores of black rust, however, show the least 
amount of impairing in their viability at laboratory tempera- 
tures because even after a month they showed as much as 
50 % germination. 
Brown rust seems to suffer more than the black but much 
less than the yellow one retaining nearly 30 % of its viability 
for a period of one month at laboratory temperatures. To 
elucidate this fact still further two more tests were made in 
April 1922 for which a constant record of temperature was kept 
with a thermograph. 
Uredospores of all the three rusts were taken fresh from 
plants inoculated on the same day and kept in the same green- 
house throughout. The temperature of the greenhouse was kept 
well within favourable limits (4:5—21° C.) and only once or 
twice rose for a few minutes to 26° C. 
I. Spores about ten days old gave the following results when 
tested at 10°5-15°5° C. (Average for twenty-four hours 12°8° C.) 
P. glumarum: nearly 50 °%% germination 
P. iriticina: about 60% ve 
P. graminis: nearly 75 % +5 
2. Spores nearly five weeks old gave the following results at 
the same temperatures as above and germinated at the same 
time. 
P. glumarum: below to % germination 
P.initicina: about 30 % a 
P. graminis: nearly 70 % yf 
These experiments show clearly that uredospores of black 
rust keep better at ordinary temperatures than either of the 
other two rusts and that those of yellow rust suffer the greatest 
loss of viability. By comparing these results with those already 
quoted one can easily see that it is not only temperature that 
is responsible for the loss of viability so obvious in the case 
