Viability of JJrediniospores 
9 
perature of 25° C. At the end of this period the plants were 
taken out, trimmed, and benched in a greenhouse equipped with 
a Johnson Service Company temperature control. During the 
period of 12 days that the plants were benched, the daily mean 
temperature of the house was 24° C. The plants were trimmed 
every third day. 
Flecking was noted on some of the plants February 5, and 
by February 7 rust pustules were beginning to appear. On 
February 14, 13 days after inoculation, the infected leaves were 
cut off and approximately 30 leaves were placed on the silk 
threads in each humidity chamber already described. 
At the end of each week, one or two leaves, depending on 
the number of uredinia present per leaf, were withdrawn from 
each dish. Duplicate germination tests were made. The re- 
mainder of the rust material was used for the weekly infection 
tests in the greenhouse. 
A composite sample of the spores from each leaf was germi- 
nated in a drop of distilled water placed on a cover glass and 
inverted over a glass ring, mounted on a glass slide. These were 
placed in a large, moist chamber and incubated for 24 hours at 
room temperature. The percentage of germination was deter- 
mined by counting a total of 200 spores at various points in the 
microscopic field. 
In the greenhouse, five seedling plants were inoculated with 
the remainder of the rust material from each dish. About 10 or 
12 days after inoculation the number of pustules per leaf was 
recorded. 
EXPERIMENTAL DATA 
Constant temperatures of 5°, 10°, 15°, 20°, 25°, and 30° C. 
were used. A set of 11 moist chambers was placed at each 
temperature, the air in each chamber being maintained at a con- 
stant relative humidity by means of sulfuric acid solutions. Thus, 
at each temperature, there was a series of relative humidities 
ranging from 0 to 100 per cent, at approximately 10 per cent 
intervals. The experiment was run over a period of 16 weeks, 
at the end of which the rust material was exhausted. 
Germination tests. — The results of the germination tests are 
given in Table 1. As none of the urediniospores held at 30° C. 
germinated, the results are not shown in the table. 
No germination of the urediniospores occurred at either the 
high or the low relative humidities at 25° C. At the medium 
relative humidities some germination took place, but the percent- 
