Apr., 1922] RAINES VEGETATIVE VIGOR OF THE HOST 193 



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 (1914, 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 (1913) 

 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 (1917, 

 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 



