226 GERMINATION OF SPORES 



and rate of germination are greatly reduced. The temperature 

 range for germination extends from 2° to 36° C. 



Thermal death point. In bacteriology the term thermal 

 death point or, more appropriately, thermal death time has b'een 

 employed to express that minimal temperature fatal to all bac- 

 teria after exposure for 10 minutes. The method used is to sub- 

 ject a suspension of bacteria to a series of selected temperatures 

 and at definite intervals to plant out portions to determine the 

 number of survivors. If the operation is repeated sufficiently 

 often, it will be found that at a particular temperature all organ- 

 isms are dead after an exposure of 10 minutes. All other factors 

 must be identical in thermal-death-time measurements, because 

 age of organisms, concentration of organisms, and pH are modi- 

 fying factors. Essentially the same method, using suspensions of 

 spores, may be employed for fungi. Smith (1923) made such a 

 study with conidia of Botrytis cinerea exposed at a range of tem- 

 peratures between 31° and 50.3° C. When he plotted the propor- 

 tion surviving at different times for each temperature, he got a 

 series of approximately symmetrical sigmoid curves all exactly 

 alike except for the rate of speed of killing at different tempera- 

 tures. If the observations at each temperature employed by 

 Smith are plotted, they will be seen to fall closely on a typical fre- 

 quency-distribution curve. 



Spores retain their viability at higher temperatures when sub- 

 jected to dry heat than to moist heat. These differences in toler- 

 ance become greater if the temperature is very slowly elevated 

 during dry heating. In explanation it may be pointed out that 

 heat coagulates proteins more readily when the moisture content 

 is high than when a small percentage of water is present. The 

 observations of Tsaugi (1933) on retention of germinabilitv by 

 oospores of Sclerospora graviinicola are concerned with this point. 

 Those subjected to dry heat at 50° C, 55° C, and 60° C remained 

 viable, whereas moist heat at these temperatures was lethal. 



Ames ( 1915) determined that the thermal death point of Thiela- 

 viopsis paradoxa is 52.5° to 53.5° C, of Rhizopus nigricans, 60° C, 

 of Monilia fructicola, 52.0° to 52.5° C, of Glomerella rufomacu- 

 lans, 53.0° to 5 3.5° C, of Cephalothecium roseum, 47° to 48° C, 

 and of Penicillin//; digitatum, 58.0° to 58.5° C. 



