30 Harold Thurston Cook 



Figure 10. germination of conidia of peronospora destructor 

 at low temperatures 



dew collected from onion leaves, and in Knop's nutrient solution 

 (figure 11). 



Murphy and M'Kay (1926) obtained germination at 8.5 to 10° C, 

 a trace at 25°, and none at 30°. They do not report temperatures lower 

 than 8.5° C, nor apparently did they determine the exact optimum 

 temperature. Katterfeld (1926) studied the germination in consider- 

 able detail, paying especial attention to the rate of growth of the germ 

 tubes, the rate of germination, and the ability of the conidia to germinate. 

 His tests were run at a laboratory temperature of 10° to 12° R. 

 (12.5° to 15° C.) and 14° R. (17.5° C), which, according to the writer's 

 experiments, are favorable temperatures for germination although a 

 little higher than the optimum. By making observations every one 

 and one-half hours Katterfeld found that at a temperature of 15° C. 

 the rate of growth of the germ tubes during the first five and one-half 

 hours varied from 46 to 111 microns and averaged 101 microns per 

 hour. Although the writer did not make a special study of the rate 

 of growth, his observations in general confirm Katterfeld 's in that the 



