Viability of TJ re cliniosp ores 
11 
age of germination decreased rapidly with an increase of the 
time interval. At this temperature the maximum length of time 
that the spores were viable, five weeks, occurred at the relative 
humidity of 49.0 per cent. 
At 20° C., the urediniospores were viable for a greater range 
of relative humidities and over a longer period than at 25° C. 
At approximately 100 per cent relative humidity, the spores were 
viable for one week. A higher percentage of germination over a 
longer period of time was obtained with each drop in humidity 
until 49.0 per cent was reached, where the spores were viable 
at the end of 11 weeks. Again, with each succeeding drop in the 
relative humidity there was a general decrease in the percentage 
of spores germinating and they remained viable for a shorter 
length of time. As at 25° C., the spores gave the highest per- 
centage of germination and were viable longer at the medium 
humidities. 
At 15° C., the urediniospores were viable over longer periods 
at all humidities than at 20° C. However, there was the same 
relation between the viability of the spores at the various relative 
humidities as at 20° C. At the relative humidities of 49.0 to 60.7 
per cent the spores germinated at the end of 10 weeks. 
At 10° C., the spores gave a higher percentage of germination 
and were viable for more extended periods at nearly all humid- 
ities than at 15° C. At the end of the sixteenth week at the 
relative humidities of 38.0, 49.0, and 70.4 per cent, a fairly high 
percentage of spores were still germinating. 
At 5° C., the viability of the spores at the medium humidities 
was still fairly high at the conclusion of the experiment. At the 
relative humidities above 70.4 per cent, the spores were viable for 
about as long as the corresponding relative humidities held at 
10° C. On the other hand, at relative humidities below 38.0 
per cent the spores were viable for a shorter time than at 10° C. 
These results show that we have reached a point where, at the 
lower relative humidities held at 5° C., the spores are not as 
viable as long as at the corresponding relative humidities at 10° C. 
If the rust material had been available for further tests, 
the spores submitted to the medium humidities and temperatures 
of 5°, 10°, and 15° C. would doubtless have remained viable for 
a considerable time, as the percentage of germination A\as still 
high at the conclusion of the experiment. 
It will be noted from the table that in the majority of in- 
stances there is a gradual decrease in the percentage of germina- 
