FOOD MAKING 



83 



synthesis has been greatly reduced by a decrease in carbon 

 dioxide content and in turn this restricts the amount of oxy- 

 gen in the atmosphere. Increased carbon dioxide has been 

 tried in greenhouses but the cost has been too high to make 

 its use profitable, in spite of increased yields. It is quite 

 possible that a cheaper source will be found so that it can be 

 profitable to increase the carbon dioxide content for some 

 crops. 



The large area from which the carbon dioxide must come 

 to supply the plant is most astonishing. If a sunflower leaf 

 derived all its supply of carbon dioxide from the air directly 

 above it, the entire carbon dioxide content would be used 

 from an eight-foot-high column of air in one hour. If all the 

 carbon dioxide used by a crop of corn were derived from the 

 air over the field about 40 per cent of the total amount over 

 the field would be used. It must be kept in mind that plants 

 (weeds or cultivated plants) cover most of the warm, humid 

 parts of the earth, using carbon dioxide at comparable rates. 



The formula on page seventy shows that plants give off 

 an amount of oxygen equal in the amount of carbon dioxide 

 used in photosynthesis. All other organisms give off carbon 

 dioxide and use oxygen in their respiration. (Plants, too, 

 give off carbon dioxide in their respiration, but only during 

 the night, in the absence of photosynthesis, do they discharge 

 it to the atmosphere in excess of its use.) When all animals 

 large and small are thought of as giving off carbon dioxide, 

 the amount seems stupendous, but it has been said that 

 plants use all this and about fifty times more, which comes 

 from fungi, bacteria, fires, volcanoes, etc. Someone has esti- 

 mated that a plant with 150 square yards of leaf surface will 

 use all the carbon dioxide given off throughout the year by 

 one man, and at the same time supply his need for oxygen 

 by its release in photosynthesis. A large tree may have 5,000 

 square yards of leaf surface. 



Garner and Allard published their original investigations, 



