EFFECTS OF TEMPERATURE ON GERMINATION 221 



fection, lack of sufficient oxygen, as Jones points out, is undoubt- 

 edly a controlling factor. 



Heavy water, deuterium oxide, as it affects germination of 

 conidia of Erysiphe gravnms tritici, was studied by Pratt (1936). 

 He varied the concentrations of D 2 from 0.02 to 100%, with 

 phosphates as buffers. Conidia germinated in all concentrations, 

 but the rate of elongation of the germ tube and its final length 

 were found to be inversely proportional to the concentration of 

 DoO. Deuterium oxide seems to limit the amount of solutes and 

 colloids within the conidia that is utilizable in growth. 



EFFECTS OF TEMPERATURE ON GERMINATION 



Temperature is known to be one of the factors that modify the 

 severity of plant diseases. It may also be the limiting factor in the 

 prevalence of diseases of crop plants in certain areas. As examples 

 it may be recalled that apple scab and late blight of potatoes are of 

 rare occurrence and are never of consequence in the Coastal 

 Plain area of the southeastern United States. Anthracnose-free 

 bean seed can be produced in portions of this area by planting at 

 such seasons that high temperatures will prevail at the critical 

 period during maturing of the crop. Blue-staining fungi are an 

 important cause of the degrading of lumber in the warmer parts 

 of the United States. 



For every fungus there is a minimum, an optimum, and a maxi- 

 mum temperature, the cardinal temperatures, for germination and 

 for subsequent growth of the fungus. The metabolic activities 

 or rate of reaction of each species increases with an increase of 

 temperature up to a certain limit. These cardinal temperatures 

 must be understood to mean both the extreme temperature limits 

 of metabolic activity, all other factors being kept constant, and 

 the temperature at which metabolism proceeds at the best rate. 



The effect of temperature on germination of urediniospores of 

 Puccinia coronata and on rate of qrowth of the s^erm tube is shown 

 by the work of Melhus and Durrell (1919), the rate being greatest 

 at the optimum temperature. At either extreme, there is no 

 growth. Rate of growth may, therefore, be regarded as a direct 

 function of (t — t°), if t represents any particular temperature, 

 and t°, the minimum temperature. In some instances, as could 

 be expected, the temperature which is optimum for germination 



