INFLUENCE OF ENVIRONMENTAL CONDITIONS 775 



2. Psychrophilic, or those which grow best at low temperatures 

 (below 10°C). 



3. Mesophilic, or those which grow best at 10° to 45°C. 

 Attention has already been called to certain processes carried on by 



thermophilic bacteria and fungi, notably the decomposition of cellu- 

 lose (p. 439). It is doubtful whether any specific group of "psychro- 

 philic" bacteria exists in the soil, these organisms being more "psychro- 

 tolerant." The presence of "thermophilic" groups of organisms is more 

 definite, although a large number of these are probably also only "ther- 

 motolerant." Most of the thermophilic microorganisms have their 

 optimum at 50 to 70°C. with a minimum at 30° to 40° and a maximum 

 at 70° to 74°C. Krohn even isolated bacteria having an optimum at 

 54° and a maximum at 89°C. The question of heating of hay called 

 forth considerable discussion. Miehe found that by inoculating hay 

 with a mixture of Bart, coli and Bac. calfactor or Oidium lactis and 

 Bac. calfactor, heating of hay to 70°C. could be brought about. At 

 that temperature, chemical processes take place in the hay which 

 lead to the formation of substances which ignite on contact with air. 

 The biological nature of the process was denied by some investigators, 

 who ascribed the phenomenon to the action of oxidative and reducing 

 enzymes. 23b 



Higher temperatures usually never become limiting factors in the 

 activities of microorganisms in the soil, except in very hot climates, 

 where only the surface layer may be affected. The destructive action 

 in this case may be due more to the lack of moisture at the surface of 

 the soil. Most microorganisms are able to withstand dryng and this 

 process exerts a decided stimulating effect upon the organisms after 

 the soil is remoistened, as shown elsewhere. The thermophilic bacteria 

 are not only able to withstand high temperatures (65° to 75°C), but 

 they may take an active part in various processes of great importance 

 in the soil (p. 157). 



It is not necessarily essential for the soil to be heated up to 45° to 

 50°C, before these organisms become active. They act in the soil 

 even at lower temperatures, but to a more limited extent. The pe- 

 culiar ability of these organisms to grow at such high temperatures is 

 due to their adaptation to environment. The temperature optimum 

 of soil bacteria depends upon the soil from which they were isolated, 



c5b Tschirch, A. Die Lokalisation der chemischen Arbeit in der Pflanze. 

 Mitt. Naturf. Gesell. Bern., 138-152. 1917; Laupper, G. Die neuesten Ergeb- 

 nisse der Heubrandforschung. Landw. Jahrb. Schweiz., 34: 1-54. 1920. 



