The Bacteria in Natural Waters. 3 



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 substances are not of such chemical com- 

 position that they can be utilized at once by green plants 

 as food, but it is first necessary that thdy go through a 

 fermentation or transformation in which their chemical 

 composition becomes changed; and it is as the agents of 

 this transformation that bacteria assume their greatest 

 importance in the world of life. 



We may take the decomposition of a comparatively 

 simple excretory product, urea, as an example of the part 

 which the bacteria play in the preparation of plant food. 

 Through the activity of an enzyme produced by certain 

 bacteria this compound unites with two molecules of water 

 and is converted into ammonium carbonate, 

 NH^ > 

 CO^ + 2 H,0 = (NH J, CO3. 



This, however, is only part of the process. While green 

 plants can derive their necessary nitrogen in part, at least, 

 from ammonium compounds, it is a well-established fact 

 that this element may be obtained much more readily 



