124 NUTRITION AND METABOLISM. 



from the death of the animal ; the toxin may be an incidental waste product 

 like the pigment. 



ROTATION OF ELEMENTS IN NATURE. 



All organic matter on earth is undergoing continuous change. Or- 

 ganisms grow and decay. The same carbon and nitrogen atoms which 

 constitute the organic world of to-day constituted it thousands of 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 are two mam fea- 

 tures in organic life, construction (anabolism) and destruction (katabolism) . 

 Construction is mainly the task of green plants, enabled by the chlorophyl 

 to use the energy of sunlight in building up organic substances 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. All urea would pass 

 gradually undecomposed into rivers, lakes, and finally into the ocean 

 where it would accumulate continuously. The enormous quantities 

 of nitrogen taken out of circulation 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, 



