Apr., 1922] RAINES — VEGETATIVE VIGOR OF THE HOST 1 93 
Similar experiments with the carnation rust (Puccinia Caryophylli on 
Dianthus sinensis) gave opposite results. The incubation period increased, 
from 15 days in January to 21 days in May. In explanation, Sheldon 
suggests the possibility that the temperature and light in the greenhouse 
were better suited to the asparagus than to the pinks. In Bailey's Cyclopedia 
of Horticulture (191 4, p. 670) the carnation is characterized as a cool-tem- 
perature plant. 
Several observers comment on the lengthening of the incubation period 
in cold weather, and the difficulty of obtaining infection in very warm 
weather. Christman (1905, p. 106) found that in the cooler weather of 
spring in Wisconsin the incubation period of cereal rusts is usually lengthened 
to between three and four weeks. Ward (1902&, p. 233) remarks that, in 
working with the brown rust of the bromes, he found infection difficult to 
carry out in hot weather; and in a succeeding paper (1905, p. 41) he re- 
peats and emphasizes the significance of the observation. In this paper 
(p- 39) » Ward also refers to experiments in which the normal development 
of the rust was interfered with by warming and chilling the root system of 
the host plant. Butler and Hayman (1906) describe unsuccessful efforts to 
produce artificial rust infection on plants growing in the open in the hot 
weather in India, and express doubts whether the uredospores have power 
to infect when exposed to temperatures exceeding 100° F. Fromme (191 3) 
found that temperatures below 20° C. increased the incubation period of 
Puccinia cor on if era on oats. 
Stakman (1914, p. 30), in his culture work with cereal rusts, observed 
the incubation period to vary with temperature conditions, both high and 
low temperatures lengthening the period very perceptibly. Mains (191 7, 
p. 187) observed that low temperatures retarded the development of P. 
coronata and P. Sorghi in the host. Stakman and Levine (1919, p. 68) 
report the optimum temperature for P. graminis tritici to be between 66.5° 
and 70° F., this giving the shortest incubation period, the most vigorous 
infection, and the largest urediniospores, for the host employed. At a 
higher temperature than 70° F. the development of the uredinia was re- 
tarded at the rate of one day for every 10 degrees' rise in temperature, but 
rust developed at as high a temperature as the host endured although the 
size of the urediniospores produced was considerably decreased. At low 
temperatures the development of the uredinia was retarded at the rate of 
one day for every 5 degrees' fall in temperature. Infection resulted at as 
low temperatures as the host could stand. The spores at the lower tem- 
peratures were rather small, but the difference was not as great as in the 
case of the high temperatures, with moderate temperatures as the basis for 
comparison. 
Lauritzen (1919, p. 19) reports experiments indicating that 42° F. is 
below the minimal temperature at which P. graminis tritici is able to infect 
wheat. Above this temperature the amount of infection rises rapidly until 
