222 



GERMINATION OF SPORES 



of spores may not be optimum for subsequent development. 

 These adaptations may be hereditary and may account for the 

 geographical distribution of the organisms concerned and for 

 their seasonal incidence. 



One type of influence of temperature upon the method of spore 

 termination was shown by Melhus (1915). He noted that the 

 sporangia of Phytophthora infestans germinated by either forma- 

 tion of a tube or formation of swarm spores. A temperature of 



30 



0) 



to 

 3 



| 20 



a 



c 

 o 



I 10 



a 



600 « 

 c 

 o 

 E 

 5 



400 g 



3 



s 



200 & 



O 



X. 



& 



c 



_3 







30 



35 



5 8 13 17 20 25 



Temperature (degrees Centigrade) 



Fig. 39. Effect of temperature upon percentage of germination and upon 

 length of cjcrni tubes in urediniospores of Viiccinia coronata. (After Melhus 

 and Durrell.) A. Percentage of germination. B. Length of germ tubes in 



microns. A close correlation is shown. 



23° C was optimum for tube formation and of 13° C for produc- 

 tion of swarm spores. These critical temperatures, as given by 

 Crozier (1933), were 24° C and 12° C, respectively. Other 

 Peronosporales, notably Peronophmnopara cnbensis, are known 

 to behave similarly. 



Cardinal temperatures for germination. From his own ob- 

 servations and those of other workers, Doran (1922) assembled in 

 tabular form data on the influence of temperature on spore germi- 

 nation among a variety of pathogenic fungi. These data consti- 

 tute the bases from which Table 19, supplemented by more recent 

 observations, has been prepared. Xo generalizations appear war- 

 ranted from these data, except perhaps that the temperatures 

 which normally prevail when the spores of these species are be- 

 in<T dispersed are favorable for germination. The data have a 

 direct bearing, however, on problems that concern the relation 



