126 APPLIED BIOLOGY 



This is constructive metabolism. On the other hand, animal 

 protoplasm cannot construct foods out of such simple ma- 

 terials ; but can only oxidize the foods made by plants. This 

 is destructive metabolism, and its result is the formation of 

 excretions. Plants do oxidize a limited portion of the foods 

 they construct, and thus they form some excretions (e.g., 

 CO 2 and H 2 0) similar to those formed when animals oxidize 

 foods. On the whole, however, plants have a limited amount 

 of destructive metabolism, and they are characteristically 

 constructive. 



Heat. It should be noted that oxidation in plant cells, as 

 well as in animals, produces heat ; but the amount is small 

 and not evident except in careful experiments. 



115. Excretions of Animals and Plants. Since oxida- 

 tion is less rapid in plants, they produce less excretions than 

 animals do. Since both animals and plants must oxidize 

 foods made by plants, it is to be expected that the excretions 

 of animals and plants will contain the same elements. As 

 we have seen, the most evident excretions of animals and 

 plants are carbon dioxide, water, nitrogenous, and mineral 

 substances. These contain all the elements which a chemist 

 could find by analyzing the cell-substance of either animals 

 or plants. 



The Cycle of Organic Matter. Since plant cells are primarily 

 constructive in their metabolism, they are able to use the 

 excretions formed by animals, which oxidize foods derived 

 from plants; or plants can use substances similar to excre- 

 tions which are formed by micro-organisms from decaying 

 animal and plant matter. Hence simple matter is constantly 

 being built into foods by plants and reduced to simple con- 

 ditions by animals. 



116. The Cycle of Carbon. Some of the carbon dioxide 

 in the air comes from the breathing of animals; some 

 comes from decaying of dead plant and animal matter; 

 and some also from the burning of wood, oil, and coal (all 



