IQ2 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 (1917, p. 189) found that the development of P. Sorghi 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 tritici 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. 



