THE ATMOSPHERE 153 



maintained. For it is constantly maintained, and the 

 question of oxygen-supply for future generations never 

 perplexes us as does the question of food-supply. Here 

 again we find one of the links which binds animal life to 

 plant life; here again we realize that we are partners in 

 life with plants, and that our lives could not continue if 

 this partnership were to be dissolved. For it is the plants 

 which, more than anything else, maintain the supply 

 of oxygen in the atmosphere. 



You have learned the process of food manufacture by 

 green plants in sunlight. The materials used in the 

 first steps of this process are water and carbon dioxide; 

 the water is obtained from the soil and the carbon dioxide 

 from the air. (Now you see why that small amount of 

 carbon dioxide in the air is so important to life.) You 

 know that in all manufacturing processes raw materials 

 are transformed into finished products, and that in connec- 

 tion with this process all of the raw material is usually 

 not consumed; the parts not consumed are wastes or 

 by-products of the process of manufacture. This thing is 

 also true of the process of food manufacture by plants; 

 they use carbon dioxide (C0 2 ) and water (H 2 0) as raw 

 materials. They use all of the carbon and of the hydrogen 

 in their finished product (carbohydrate food), but they 

 do not use all of the oxygen, and this excess of oxygen is 

 returned to the air. It is a sort of waste or by-product 

 in food manufacture, but it serves the very important 

 purpose of keeping up the supply of oxygen which is needed 

 for breathing. When you think that nearly all the land 

 surface of the earth is covered, at least half the time, 

 with green plants, and that this process goes on in all of 

 them while the sun shines upon them, you can readily see 



