112 PLANTS AND MAN 



respiratory phase of their metabolism, green plants use up definite 

 amounts of carbon dioxide and nitrates in their synthesizing 

 activities while manufacturing food. Why does not the supply of 

 oxygen, carbon dioxide and nitrates ever become exhausted? 

 The answer to this reveals a natural cycle of the elements in 

 question, in which organisms are mutually very helpful — one 

 organism returning to the environment what another organism 

 removes from it. 



Oxygen is essential in order to have respiration continue; 

 without respiration as a means of liberating the potential energy 

 of food all multicellular life would be eliminated from the face 

 of the earth. Since animal species are continually removing 

 oxygen from the atmosphere, it would not be long before all the 

 available oxygen would be used up. That this does not happen 

 is due to the fact that when green plants are carrying on photo- 

 synthesis, oxygen is released as a by-product (see p. 16). 

 Green plants are the depositors, animals the chief withdrawers, of 

 oxygen in the cosmic bank account. Our forests and meadows are 

 therefore more than so many feet of lumber or so much fodder for 

 cattle. They keep the air continually fresh by renewing the 

 oxygen supply at the same time they are removing the carbon 

 dioxide which, if accumulated in quantity, would in itself bring 

 an end to all animal life on the earth. 



Importance of Decay Bacteria 



Other relatively unknown and unappreciated inter-relations 

 of plants with animals involve a new kind of eternal triangle; 

 in this case, green plants, animals and bacteria. As plants and 

 animals grow and reproduce, generation after generation, the 

 increase in the bulk of protoplasm thus produced is tremendous; 

 and since protoplasm is made up of definite elements, the 

 increase in volume of protoplasm is proportionate to the decrease 

 in the amount of the substances used up in making that proto- 

 plasm. Two elements in particular — carbon and nitrogen— are 

 thus being continually withdrawn from the environment to be 

 made into protoplasm; the source of the former being carbon 

 dioxide, of the latter, nitrates. It is not difficult to realize that, un- 

 less the carbon and nitrogen can eventually return to the environ- 



