514 FROMME: THE CULTURE OF CEREAL RUSTS 
under which the rust might become self-propagating by close 
association of cultures, a sash frame culture box, 3 ft. square, was 
made of five window sash. The humidity maintained here when 
cultures were growing in it was quite constant and averaged 93 per 
cent. with occasional fluctuations of 2-3 per cent. Even insucha 
humid atmosphere new infections occurred only sparingly, al- 
though cultures were sprayed and heavily inoculated. To obtain 
good infections it was necessary to cover the cultures with bell 
jars as in the open greenhouse. 
_ A direct comparison of the effects of covering and not covering 
in the culture box was obtained by a statistical study. Two 
pots of seedlings, each seven days old, were inoculated simul- 
taneously from the same culture, after which one was covered for 
twenty-four hours and the other was left uncovered in the culture 
box. During the remaining eight days of the incubation period 
they were exposed to equal conditions of humidity. The differ- 
ence in the degree of infection obtained on the two cultures was 
covered culture if that obtained on the covered is regarded as the 
normal. The difference in the degree of humidity to which the 
cultures were exposed for twenty-four hours after inoculation could 
not have been more than 7 per cent, as the average in the culture 
box was 93 per cent and the atmosphere under the bell jar was _ 
presumably saturated. It is rather striking that this difference 
of 7 per cent should have produced a difference in degree of infec- 
tion of 94 per cent. The spores that produced the 6 per cent 
normal infection on the non-covered culture probably germinated 
more rapidly or were more favorably located with reference to 
moisture than the bulk of the spores. 
Effect of temperature 
To test the effect of different temperatures on the degree and 
rate of development of P. corontfera, two cultures of the same 
age were inoculated simultaneously from the same stock culture. 
Immediately afterwards one was placed in the greenhouse ‘‘stove”’ 
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