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AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
in the dry soil, while the mesophytic types showed slightly greater virulence 
of disease in the moist soils. Repeated trials were made with substantially 
the same result. Stakman concludes: . 
It is probable then that, conditions having been favorable for a rust infection, the water 
relation in the soil which is most favorable for the host plant's development is also the most 
favorable for the development of the rust. 
Mains (191 7, p. 189) found that the development of P. SorgM on corn, 
as shown by the number of pustules produced, is favored by a humid at- 
mosphere and by a wet soil — conditions favorable to the growth of the 
corn plant. The length of the incubation period was not appreciably 
influenced. 
Stakman and Levine (1919, p. 45), in experiments to determine the 
length of time that wheat seedlings inoculated with P. graminis tritici 
should be kept in a saturated atmosphere in order to obtain maximum in- 
fection, found that keeping the plants under a bell jar for more than 48 
hours reduced the amount of infection obtained and appreciably lengthened 
the incubation period. In other experiments (p. 70) they noted a tendency 
for excessively high or excessively low humidity during the incubation 
period to cause a decrease in the size of the urediniospores. In another 
experiment on soil moisture (p. 71) in which three series of plants were 
employed, one of which was heavily watered, the second moderately, and 
the third received only enough water to prevent the plants from wilting, 
Stakman and Levine found that the plants in the wet soil were more severely 
attacked and that the urediniospores developed on them were larger than 
those in the other two series. The plants that suffered from drought pro- 
duced the smallest spores. The authors conclude as a result of their study 
on the effect of environmental factors on the morphology of the uredinio- 
spore of Puccinia graminis triiici that deficiency of soil moisture and of sun- 
light and other ecological factors affecting the host plant unfavorably ap- 
pear to be equally unfavorable to the rust parasite. 
Temperature Relations 
There is abundant evidence of a tendency towards physiological par- 
allelism of host and rust in their temperature relations. 
Sheldon (1903, p. 33) studied the relation between greenhouse tempera- 
ture and hours of sunshine per day, and the length of the incubation period 
in the asparagus rust. His experiments extended over a period of five 
months, from December, 1900, to May, 1901, and yielded data on 132 
asparagus plants. The results indicate an inverse relation between the 
temperature and light conditions under which the host was growing and the 
incubation period of the rust. During December and January the length 
of the incubation period was regularly 14 to 17 days. During April and 
May, when the day was longer, the light better, and the temperature 
higher, the length of the incubation period was only 8 to 10 days. 
