184 E. B. MAINS 
Considerable difference of opinion has arisen concerning the effect 
of moisture on the development of the rusts. De Bary (1863) found 
that vegetative development of Uromyces appendiculatus and its pro- 
duction of spores was greatly increased by a humid atmosphere. 
McAlpine (1906) reported that drainage increased the yields of wheat, 
but did not decrease the rust. He also considered that irrigation late 
in the season tended to make the grain soft and brought on the rust. 
Stone and Smith (1899) and Blaringhem (1912) claimed that the rusts 
were favored by dryness. Sirrine (1900) and Buchet (191 3), on the 
other hand, claimed that the rusts were favored by wet soils. Smith 
(1904) considered that a dry atmosphere retarded the development of 
the rust within the host while a dry soil favored development. 
Fromme (191 3) found that for Puccinia coronata after infection has 
taken place moisture appears to have no effect upon the length of the 
incubation period. 
The effect of temperature upon the development of rusts in their 
hosts has been but little studied. Sheldon (1902) found that the 
incubation period of Puccinia Asparagi was longer at an average 
temperature of 69° F. during the winter months than at an average 
temperature of 76° F. during the summer. It is likely that these 
results were in part due to the difference in the amount of light present 
in the two seasons. Fromme (191 3) discovered that for Puccinia 
coronata a temperature between 20° and 30° C. brings about a shorter 
incubation period than a temperature of 14° to 21° C. 
Fromme (1913) appears to be the only one who has definitely in- 
vestigated the effect of light upon the development of the rusts. He 
found that when oats inoculated with Puccinia coronata were placed 
in darkness for a few days and then returned to the light, the incuba- 
tion period was lengthened approximately by the time that the oats 
were in the dark. Fromme states that this may mean a dependence 
of the rust upon the transition products of photosynthesis and that 
this may explain the obligate parasitism of the rusts. 
III. EXPERIMENTAL WORK 
MATERIAL 
Two rusts were employed, Puccinia coronata Cda. and Puccinia 
Sorghi Schw. Puccinia Sorghi was used in all of the experiments since 
its host maize {Zea Mays L.) was easier to work with. Puccinia 
