NUTRITION OF MICROORGANISMS 259 



and we find that the destructive work is to a large extent the task of 

 microorganisms. Many products of organic life cannot be broken 

 down by organisms other than bacteria, and therefore bacteria are 

 absolutely necessary for the circulation of the elements and for life on 

 earth. Bacteria and green plants are an absolute necessity for the 

 maintenance of life, the one breaking down, the other building up, 

 one dependent upon the products of the other; animals, however, could 

 be excluded from the circle without interfering with a continuation of 

 life on earth. 



ranisms 



Carbohydrates 

 /'at, frotein 



FIG. 114. Carbon cycle. 



CARBON CYCLE. Carbon is the main element in organic nature, and 

 the study of its cycle might be begun with its simplest compound, 

 the carbon dioxide of the air. It is absorbed in this condition by the 

 green plants, and is changed by the chlorophyl granules of the leaves to 

 organic compounds of various types, either to carbohydrates (cellulose, 

 starch, sugars) or to fats, or to protein substances, occasionally to 

 organic acids or other compounds. The plants will either die and decay, 

 or will be eaten by animals. In the first case, the decay will be caused 

 exclusively by microorganisms; if the plants are eaten, they will be 

 digested; part may be used to build up the animal body or stored as 



