HABITATS AND LIFE CONDITIONS 145 



tically all air at low levels, except over mid-ocean, 

 contains more or less dust. Dust is carried about in 

 currents of air, and is often blown by the wind for very 

 long distances. When the air becomes comparatively 

 quiet, the particles drift slowly down and settle. Any 

 living cells which it contains, vegetative bacterial 

 cells, bacterial spores or fungal spores, will grow if 

 they find themselves in suitable conditions of moisture, 

 food supply and temperature. In such conditions 

 spores will germinate, and such vegetative cells as have 

 not been killed by drying up will grow and divide. 



Habitats and Conditions of Life. This is why all 

 dead bodies exposed to air putrefy and eventually fall 

 to pieces, provided they remain moist and not too 

 cold ; and why all milk, beer or wine similarly exposed 

 turns sour. These processes all depend on the activity 

 of the living protoplasm of bacteria. Each is carried 

 out by particular species only, and cells or spores of 

 some of these are always found in the dust of the air, 

 so that they are certain to fall sooner or later on to the 

 surface of the dead body or into the organic liquid. 

 These processes go on more rapidly at fairly high 

 temperatures, simply because such temperatures are 

 the most favourable for bacterial growth and meta- 

 bolism ; and they are suspended at o C. or below 

 (though the bacteria are not killed), because protoplasm 

 cannot work at the freezing-point of water. 



In order to feed, live and multiply, each species of 

 bacterium must have the appropriate conditions for 

 that species, the right sort of food, the presence (or 

 absence) of oxygen, a sufficiency of water in the medium, 

 and a suitable temperature. The temperature relations 

 of bacteria are remarkably wide and various. Most 

 species live at the ordinary temperatures which prevail 

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