TEMPERATURE 53 



temperature produced by the sudden evaporation of liquid 

 hydrogen (—256° to —265°) and have remained alive. 

 Whether they could withstand such temperatures indefinitely 

 is not known. The minimum for development is near the 

 freezing-point of water, since reproduction by division has 

 been observed in the water from melting sea-ice at a tem- 

 perature of — 1.5°. Thus bacteria as a class have a range 

 for existence of about 373° (—273° to + 100°) and for develop- 

 ment of 94.5° (—1.5° to + 93°), certainly much wider ranges 

 than any other group of organisms.^ 



The optimum temperature for development varies within 

 rather wide limits for different organisms. In general it 

 may be stated that the optimum temperature is approxi- 

 mately that of the natural habitat of the organism, though 

 there are exceptions. The optimum of the "hot spring" 

 bacteria just mentioned is apparently that of the springs 

 (93° in this case). Many soil organisms are known whose 

 optimum is near 70° (a temperature rarely, if ever, attained 

 in the soil) when grown m air or oxygen, but is very much 

 lower when grown in the absence of oxygen. Many other 

 soil organisms exhibit very little difference in rate or amount 

 of growth w^hen grown at temperatures which may vary 

 as much as 10° or 15°, apparently an adaptation to their 

 normal environment. The disease-producing organisms 

 show^ much narrower limits for growth, especially those 

 which are difficult to cultivate outside the body. For 

 example, the bacterium of tuberculosis in man scarcely 

 develops beyond the limits of 2° or 3° from the normal 

 body temperature of man (37°), while the bacterium of 

 tuberculosis in birds grows best at 41° to 45°, the normal 

 for birds, and the bacterium of so-called tuberculosis of cold- 

 blooded animals at 14° to 18°. 



Those bacteria whose optimum temperature is above 40° 

 are sometimes spoken of as the '' tliermophiV bacteria. The 

 fixing of the "thermal death-point," that is the minimum 



1 With the possible exception of blue-green algae which have been found 

 with bacteria in the above-mentioned hot springs. Seeds of many plants 

 have been subjected to as low temperatures as those above mentioned with- 

 out apparent injury. 



