PRODUCTS OF METABOLISM 183 ' 



years ago. The amount of carbon, nitrogen, hydrogen and of all 

 other elements of life on earth is limited, and the same atoms will 

 be used for the future generations of life that constitute the present. 

 There must be continuous destruction to enable new construction. 

 Construction is mainly the task of green plants, enabled by the chloro- 

 phyl to use the energy of sunlight in building up organic substjtnces 

 from minerals, water and carbon dioxide. Destruction is caused 

 mainly by animals and other organisms which have to break down 

 organic matter in order to exist. These two factors keep the atoms of 

 the organic world in perpetual rotation. 



In this circulation of the elements it is necessary that all compounds 

 of organic nature be decomposed finally to a form available for plant 

 food. If this were not the case, the indestructible compound would 

 sooner or later accumulate in such enormous quantities that the 

 elements constituting this body would be removed entirely from 

 general circulation. Let us suppose, as an illustration, that for some 

 unknown reason, all urea bacteria on earth would die. Urea could be 

 decomposed no more, and the plants, unable to use urea as a source of 

 nitrogen in place of nitrates, would get but little benefit out of stable 

 manure. AH urea would pass gradually undecomposed into rivers, 

 lakes, and finally into the ocean where it would accumulate con- 

 tinuously. The enormous quantities of nitrogen taken out of cir- 

 culation would cause a decreasing growth of plants, and life would 

 soon cease because of lack of nitrogen. For this reason all products of 

 living organisms must be further broken up by some other organisms, 

 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. 



Caebon 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 



