4 ELEMENTS OF WATER BACTERIOLOGY. 



First, then, the bacteria, though possessing greater con- 

 structive power than any animal organism, lack the power 

 of green plants to build up their own food from purely 

 inorganic materials, and must live upon the products of 

 the growth of higher forms. A few species which have 

 become adapted to a parasitic or semi-parasitic mode 

 of life occur on the surface of the normal plant or animal 

 body or penetrate the deeper layers of diseased tissues, 

 feeding upon the fluids of the body or on the extraneous 

 material collected upon its surface. Even these bacteria, 

 however, may generally be cultivated under saprophytic 

 conditions, and the vast majority of other forms live as 

 true saprophytes on dead organic matter wherever it 

 may occur in nature,, and particularly in that diffuse layer 

 of decomposing plant and animal material which we call 

 the humus, or surface layer of the soil. Wherever there- 

 is life, waste matter is constantly being produced, and 

 this finds its way to the earth or to some body of water. 

 The excretions of animals, the dead tissues and broken- 

 down cells of both animals and plants, as well as the 

 wastes of domestic and industrial life, all eventually find 

 their way to the soil. In a majority of cases these sub- 

 stances are not of such chemical composition that they 

 can be utilized at once by green plants as food, but it is 

 first necessary that they go through a fermentation or trans- 

 formation in which their chemical composition becomes 

 greatly changed; and it is as the agents of this trans- 

 formation that bacteria assume their greatest importance 

 in the world of life. 



