4?6 
Journal of Agricultural Research voi.xxvn, no . 7 
infected plant recorded is one which had been transferred. It is to be 
remarked that, although the percentage of infection is higher in the 
plants transferred to high temperatures than in those exposed constantly 
to high temperatures, the number of infected plants is much lower than 
in those instances in which transfers were made to the lower temperature 
series. 
A histological study of infected seedlings has shown that a period of 
six days at 15 ° C. is longer than is required for entrance of the parasite 
under the conditions mentioned above; hence, it is highly probable that 
the germ tubes had entered and became established in the plants before 
they were transplanted. The subsequent decrease in the percentage of 
recorded infection is due either to the direct action of temperature or to 
changed physiologic conditions in the host plant. 
Walker and Jones (5^), in discussing the effect of temperature on the 
development of onion smut, report that “exposure of onion plants bear¬ 
ing incipient infections to a temperature of from 30° to 33 0 C. for from 
12 to 15 days almost entirely checked further development of the para¬ 
site.” High air temperatures alone were insufficient to check develop¬ 
ment of the disease, and it was suggested that the inhibitory effect noted 
above “may be due in part at least to the influence of the environmen¬ 
tal conditions upon the metabolism of the host and not entirely to a 
direct effect upon the fungus itself.” 
In the experiments with flag smut, the roots and the small basal por¬ 
tion of the shoot only were exposed to the constant temperature ranges. 
There is no direct evidence that a change in physiological condition of 
the host was responsible for checking the amount of infection at the 
higher temperatures, for it is possible that, as the young tissues of cen¬ 
tral portions of the plant must have been influenced directly by tem¬ 
perature, the fungus also must have been subject to the same influence. 
Laboratory studies have shown that constant exposure to tempera¬ 
tures above 24 0 C. is unfavorable to the development of the fungus, and 
it would appear that in pots 12 and 16 the plants were more tolerant of 
the higher temperatures than was the parasite. 
Under the conditions of the experiment, there appeared to be no signifi¬ 
cant differences between presoaked and dry spores when used as inoculum. 
In these cases the only evidence as to the degree of penetration of the 
fungus is that derived from a consideration of the degree of infection. It 
appeared that in the majority of instances in which ungerminated spores 
were used for inoculation at low temperatures, the organism was less 
successful in reaching the tissues in the region of the growing point, and, 
in a number of instances, the main shoots escaped,entirely. It appeared 
that the organism persisted in the lower nodes and thus was readily able 
to infect the young shoots which arose from them. 
Previous experiments have shown that the spores will germinate at 
5 0 C. A test was made to determine if infection might occur at this 
temperature. Twenty seedlings of Canberra wheat, in which the coleop- 
tiles had just appeared, were kept at 5 0 for several hours, then heavily 
inoculated with germinating spores of Urocystis tritici. They were then 
kept in a moist atmosphere at 5 0 for seven days after which they were re¬ 
moved, thoroughly washed, heavily dusted with copper carbonate, and 
then planted in sterilized soil. Six weeks later 5 per cent of the plants 
had produced small culms which bore characteristic lesions of flag smut. 
None of the plants showed any sign of infection on the main stem. They 
were allowed to head, but no further sign of the disease appeared. 
