528 



BIOLOGY FOR BEGINNERS 



However, the carbon is not lost. When either plant or animal, 

 fire or decay, oxidize these plant products, carbon dioxide is set 

 free again in the same amounts as before, mixes with the at- 

 mosphere, and is ready for plant use again. No atom of carbon has 

 ever been destroyed or produced by life processes; it is merely 

 used over and over again. 



The Oxygen Circle. Oxygen is equally important, both as 



5ALT5(Ca, ria, P, F t ,n^, 5, 

 NITRATES . NITRITES 



NH CO NITRATES 



' PHOSPHATES 



OTHER SALTS 



EARTH -"WATER 



FIG. 160. Diagram illustrating the cycle of living matter and energy in 

 animals, plants, yeast and bacteria. From Calkins. 



being a part of all living tissue, and as the liberator of vital energy. 

 It is taken from the air whenever plants or animals breathe, or 

 wherever fire burns or substances decay. 



All these processes combine the oxygen into carbon dioxide, 

 water, or other oxides, and one might suppose that it was per- 

 manently removed from circulation, but this is not the case. 

 Plants take this carbon dioxide and water, unite them to form 



