Feb. 16, 1924 
Flag Smut of Wheat 
469 
humidities of o, 10, 25, 50, 75, 90, and 100 per cent were secured by the 
use of sulphuric acid in a series of dilutions, according to the method 
described by Wilson (56). The solutions were placed in wide-mouthed 
bottles. Infected leaves were cut into sections about 5 mm. in length, 
and were then suspended over the solutions in small paraffined containers 
fastened to the undersurface of the corks, the latter being sealed down 
with paraffin. The bottles were placed in incubators at the following 
temperatures: 5 0 to 7 0 C., 18° to 21 °, 26.5° and 37 0 . The results of early 
experiments are not recorded, as the spores did not germinate readily in 
any known nutrient solution. The spore material used in the following 
experiment had been kept for eight months in the laboratory prior to 
the commencement of the experiment. Germination tests were made at 
approximately monthly intervals over a period of five months and, on each 
occasion, small portions of wheat-seedling tissue were used to stimulate 
the germination of the spores in water by the method already described. 
Under the conditions of the experiment there was no marked difference 
in the amount of germination at each relative humidity within a tem¬ 
perature range of 5 0 to 26.5° C. At 90 and 100 per cent relative humidi¬ 
ties, however, it became difficult to separate the spores from the host 
tissue. Consequently, there was a greater amount of contamination in 
these series and the results are not entirely comparable with those ob¬ 
tained in the lower relative humidities. It was observed also that, even 
after the tissue had been exposed to 100 per cent relative humidity for 
five months at the temperatures ranging from 5 0 to 22 0 , sometimes 
almost all the spores of certain small batches germinated. 
The consolidated figures for the percentage of germination at all rela¬ 
tive humidities for each of the three lower temperature ranges (5 0 to 
26.5° C.) indicate a definite relationship between relative humidity and 
the viability of the spores (Table VI). Seventy-five per cent relative 
humidity appeared to be the optimum for the retention of viability at 
these temperatures. The figures obtained for the 50 per cent relative- 
humidity series indicate that this also is very favorable for the retention 
of viability. At both higher and lower humidities, comparatively few 
spores germinated toward the end of the experiment. This was espe¬ 
cially true in all cases at the lower humidities under the conditions of the 
experiment. 
Table VI .—The effect of humidity on the viability of spores of Urocystis tritici eight 
months old at commencement of experiment on November 22 
Approximate relative 
humidity. 
Relative germination on—“ 
Totals 
for each 
relative 
humidity. 
Dec. 13. 
Dec. 29. 
Jan. 31. 
Feb. 27. 
Mar. 25. 
Apr. 25. 
O. 
86 
85 
16 
4 
3 
8 
202 
IO. 
68 
X 5 2 
22 
11 
7 
33 
2 93 
2 5 . 
6 
* 3 * 
75 
105 
22 
55 
394 
50 . 
192 
177 
196 
236 
149 
148 
1, 098 
75 . 
261 
208 
207 
2 43 
202 
185 
1,306 
90. 
*57 
2 5 
5 i 
6 
63 
68 
37 0 
100. 
IT 4 
158 
43 
5 
3 1 
6 
357 
Totals for all 
temperatures 
and all hu¬ 
midities . 
884 
93 6 
610 
610 
477 
50.3 
a Totals of percentage of germination at each relative humidity for three temperature series (5 0 to 26.5° C.). 
