TEMPERATURE 



100 



411 



Time, hours 



Figure 8. Temperature and the rate of germination of uredospores of Phragmidium 

 mucronatum. Redrawn from Cochrane (57), by permission of the Cornell University 

 Agricultural Experiment Station. 



reactions occurring in the very earliest phases of germination, is uni- 

 formly extremely sensitive to low temperature. At supraoptimal tem- 

 peratures there is again evident a differential sensitivity of these early 

 reactions (284). 



Sensitivity to low temperature of the processes concerned in the ini- 

 tiation of germination is paralleled by sensitivity of the same processes 

 to a favorable temperature. The germination of uredospores of Phrag- 

 midium mucronatum at non-optimal temperatures is markedly accel- 

 erated by exposure for a short time to a moderate temperature; for 

 example, germination at 9°C is increased about 160 per cent by a pre- 

 exposure for one hour at 18°, during which exposure there is no visible 

 sign of germination (58). The same phenomenon in Puccinia chrysan- 

 themi is suggested by the data of Campbell and Dimock (44). Simi- 

 larly, direct germination, by germ tube formation, of the conidiosporan- 

 gia of Phytophthora infestans at 20° is greatly increased by prior 

 exposure for 5-10 minutes to 40° (279). 



It can be suggested, therefore, that early metabolic reactions in ger- 

 mination have a different and more sensitive response to temperature 

 than do the later stages. Obviously, the evidence is far from complete. 

 It is perhaps analogous that the cracking of the sporangium wall in 

 Blastocladiella sp. responds somewhat differently to temperature than 

 do the later stages of germination (45). 



Parenthetically, it may be noted that these results have a methodo- 



