652 General and Applied Biology 



thesis to manufacture more complicated plant proteins and fats. Ani- 

 mals use these carbon materials of plants with which to build even more 

 complicated animal carbon compounds. 



When plants and anim.als die, their carbon compounds are reduced 

 into simpler carbon materials which can eventually be used again by 

 living plants. Earthworms, certain insects, and such plants as bacteria 

 and molds aid in restoring this carbon material to the soil where it can 

 again be utilized. 



3. Oxygen Cycle. — Oxygen is also an essential constituent of proto- 

 plasm (Fig. 325). It not only goes into the make-up of the protoplasm, 

 but it is used in the process of oxidation, in which the oxygen combines 

 with a substance so as to liberate the energy which originally held the 

 units of the substance together. Oxygen is liberated and carbon dioxide 

 is taken in by green plants during the active process of photosynthesis. 

 Some of the oxygen is retained by the plant and used for building or 

 oxidation purposes. Animals require oxygen for respiration. This oxy- 

 gen oxidizes the foods of the animal with the release of energy and the 

 formation of carbon dioxide which can be utilized again by chlorophyll- 

 bearing plants. Hence, there is a mutual exchange and interdependence 

 between animals and plants as far as oxygen is concerned. This ex- 

 change of oxygen and carbon dioxide also occurs between animals and 

 plants in an aquarium, out-of-door pool, or any other body of water. 



D. Biologic Communities (Associations) and Successions of Plants and 

 Animals 



Very few, if any, organisms live alone (Fig. 326). In many instances, 

 groups of the same or different species are associated in a community 

 in which there may be unity, disunity, helpfulness, interdependence, or 

 destruction, depending on the many factors or conditions under which 

 they are living. A certain association of organisms m.ay live in harmony 

 in one community with its particular environments, while the same asso- 

 ciation of organisms in another community might not live harmoniously 

 because of certain environmental factors which differ from those in the 

 first community. There is not one, all-important factor which is re- 

 sponsible for the distribution and successful living of animals and plants 

 in any community. In making a scientific study of the reasons why 

 certain organisms live as they do, we must take into account the he- 

 redity of those organisms, as well as such environmental influences as 

 chemical, physical, and biologic factors in the surroundings. Even hu- 

 man factors may be quite influential as will be observed when these are 



