RELATION OF SOME RUSTS TO THEIR HOSTS 1 89 
was kept saturated with water. The other five were watered just 
enough to prevent the plants from wilting. The humidity of the room 
varied between 20 and 36 percent and the temperature averaged 
24° C. At the end of nine days after inoculation the number of pus- 
tules on the plants was counted. 
The plants in dry air and moist soil averaged 51 pustules per plant. 
These pustules, however, were small and not as well developed as in 
the others. Plants in a dry atmosphere and wet soil averaged 151 
pustules per plant. These pustules were large and well filled with 
spores. Plants in a saturated atmospheric and wet soil averaged 371 
pustules per plant. These pustules were large and difficult to count 
as many of them had become confluent. 
At the end of 25 days, the infected leaves of plants in dry air and 
moist soil were all dead and dried up, and the new leaves were free 
from the rust. The plants in dry air and wet soil had a few pustules 
on a few old live leaves, but most of the infected leaves were dead. 
The new leaves upon these plants were free from the rust. Upon the 
dry, dead leaves of these plants green areas surrounding the pustules 
were still evident. The tissue surrounding these areas had the brown 
appearance of tissue whose cells had disintegrated before they had 
dried. This would indicate that the green areas surrounding the 
pustules had died because of the drying out rather than because of the 
effect of the rust. The plants in wet soil and saturated atmosphere 
still had a number of live leaves heavily infected with rust. Some of 
the leaves however were dead or dying. On the latter, the infected 
areas remained green surrounded by yellow, sickly tissue or by brown 
tissue composed of dead cells. The new leaves had a small number of 
pustules showing that by this time some reinfection had taken place. 
The development of Puccinia Sorghi, as shown by the number of 
pustules produced, is thus favored by the saturated atmosphere on 
the one hand and by the wet soil on the other. The length of the 
incubation period, however, is not much influenced. In dry air, the 
plants finally become free from the rust by the drying up of the infected 
leaves; and reinfection does not take place since spore germination is 
prevented in dry air. In a saturated atmosphere, the infected leaves 
live for a longer time since they do not dry up; and reinfection also 
takes place to some extent, since the spores produced are able to 
germinate in the humid atmosphere. 
