THE BACTERIA IN NATURAL WATERS 3 



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 composition that they can be utilized 

 at once by green plants as food, but it is first necessary 

 that they go through a decomposition or transforma- 

 tion in which their chemical nature 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 pro- 

 duced by certain bacteria this compound unites with 

 two molecules of water and is converted into ammonium 

 carbonate, 



+ 2H 2 = (NH 4 ) 2 C0 3 . 

 NH 2 



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 is often obtained 

 more readily from nitrates, and there are other bacteria 

 which as a further step oxidize the ammoniacal nitro- 



