BACTERIA IN NATURAL PROCESSES 



65 



What has become of what was once an animal or a tree ? The 

 answer is the explanation of nature's perpetual youth: no part 

 of that disintegrating mass is lost ; all is again utilized in one form 

 or another. The greater part is transformed by bacteria into 

 substances that can be used by plants as food. The fact that 

 Nature works in a cycle, using the same material over and over 

 again, first by the plant and then by the animal and then again 

 by the plant, explains her seemingly limitless supply. 



A well-known phase of the interdependence of plants and ani- 

 mals is the fact that animals during respiration take in oxygen 

 but exhale it again in combination with carbon ; on the other hand, 

 plants draw into their leaves carbon dioxide, retain the carbon, 

 and exhale the oxygen. 



Nitrogen Cycle. A similar but more complex cycle occurs with 

 all the other elements of plant and animal life. Substances are 



.ifion 



Free 

 NitroG-er* 



1C 



acterja 



Soil Nitrates 



FIG. 20. Nitrogen Cycle. 



carried through a series of changes and in their transition furnish 

 the necessary energy for the life work of the individual whether 

 plant or animal, and finally they are returned to approximately 

 the same form again, to start once more on their journey. 



The food cycle is partially represented in Fig. 20. The air 

 furnishes the plant with carbon dioxide and water and the soil A 

 with the remaining ingredients. Of the latter the nitrogen com- 



