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E. B. MAINS 
surrounding dead tissue naturally withdraws water from the green 
infected areas, which have their water supply from the root diminished, 
and brings about death through drying out. 
If conditions at this time are unfavorable for spore germination, 
such as a low humidity or a too low or too high temperature, the corn 
plant will be freed from the rust, since the spores formed upon the 
old leaves will not be able to infect the young newly formed leaves. 
With the death of the old leaves the host becomes free from the rust. 
In the same way, oats may become free from their rust. 
The work upon the effect of temperature upon the development of 
the rust also throws some light upon the relation between the rusts 
and their host. From Experiments i and 2 and especially 3 and 4, 
it is evident that in a saturated atmosphere the development of both 
Puccinia coronata and Puccinia Sorghi is retarded by low temperatures. 
It is difficult to say just how much these results are due to the 
direct effect of the temperatures upon the rust, since the rust must be 
studied in connection with its host. A search of the literature shows 
that but little work has been done upon the effect of temperature upon 
the growth of parasitic fungi in their hosts. Sheldon (1902) found that 
during the winter months the incubation period of Puccinia Asparagi 
was longer than during the summer months when the temperature 
was higher. Fromme (1913) found a shorter incubation period for 
Puccinia coronata at temperatures between 20° and 30° C. than at 
lower. Ward (1902&) explains the non-infection in some of his experi- 
ments by the high temperature, although the host seemed to be un- 
harmed. 
The effect of temperature upon the development of saprophytic 
fungi has received considerable attention. In such experiments, 
however, the nutrient media remained unchanged. In experiments 
with the rusts, other conditions besides the temperature alter, since 
the physiological conditions in the host are altered. It is consequently 
hard to determine how much of the effect produced on the rust is due 
directly to the temperature. Lehenbauer (1914) has shown that for 
corn the optimum temperature for the growth was situated between 
29 and 32° C. Sachs (1882) gives 27.2° C. as the optimum tempera- 
ture for the growth of the root. Besides the effect upon the growth, 
two of the physiological processes of the host are especially affected. 
These are respiration and photosynthesis. The respiration of plants 
increases with the increase in temperature until the injurious effect of 
