CHAPTER XXX 

 How PLANTS TAKE FOOD FROM THE AIR 



WHILE the soil supplies plants with, nitrogenous 

 food, the air supplies them with carbonaceous food. 

 Carbon is the principal element. It exists in the 

 air in the form of carbon dioxide, or carbonic acid. 

 The quantity of this gas in the air is comparatively 

 small and ever varying in amount. It is yet suffi- 

 cient at all times to furnish a bountiful supply of 

 this kind of food for the whole vegetable kingdom. 



The leaves of the plant are the organs which 

 extract this food from the air. Each molecule of 

 the gas is composed of one atom of carbon united 

 with two atoms of oxygen. Its chemical symbol 

 is therefore CO 2 . The gas brings into contact with 

 the surface of the leaf its millions of molecules. 

 The leaf acts as the engine. The sunbeam is the 

 motor power. The leaf, thus run, breaks up the 

 molecule, seizes the atom of carbon and appropri- 

 ates it as food. The two atoms of oxygen, thus 

 liberated, go free into the air. 



This is illustrated in Fig. 67. The Clusters of the 

 dark and red dots represent molecules of carbonic 

 acid. The dark dot in each molecule is the atom 

 of carbon. The two red dots in each molecule are 

 the two atoms of oxygen. These molecules float 



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