BACTERIA. 15 



soil unless the supply was continually replenished. 

 Now, the source of carbon and nitrogen is the excre- 

 tions and secretions of animals, which contain these 

 elements in combination with other elements. By the 

 action of bacteria the complex animal matter is de- 

 composed into the chemical elements that compose it. 

 In this way the plants derive their. carbon and nitro- 

 gen from the soil. Within the body the bacteria carry 

 on much the same activities. The digestion and ab- 

 sorption in] the intestine is dependent to a large extent 

 on the breaking-down action of bacteria. We cannot 

 absorb meat and vegetable as such, and it is only after 

 our food has been separated into simple compounds 

 and elements that it is absorbed to nourish the body. 

 In this process the bacteria play no> small part. But 

 bacteria are not only agents capable of breaking down 

 complex substances; they also build up substances 

 from chemical elements. Some plants take their nitro- 

 gen from the air, but they would not be able toi do so 

 were it not for the presence of certain bacteria grow- 

 ing* in the roots. 



The maintenance of life in the world is often 

 described as a cycle; first, the chemical elements are 

 built up into plants, the plants nourish the animals, 

 then the animal tissue is consumed and excreted to be 

 broken down into elements. In each step the bacteria 

 play a most important part. 



These activities of bacteria and their enzymes are commer- 



t /. . ... . cial use 



made use of commercially; the fermenting action on of bac- 

 sugars converting them into alcohol is used in making 



