PHOTOSYNTHESIS: GENERAL 27 



annually seventy million tons of carbohydrate material, requir- 

 ing one hundred fourteen million tons of carbon dioxide. One 

 ounce of starch yields 116 calories when burned. Therefore, if 

 all this wheat were burned, the heat produced would be sufficient 

 to raise to the boiling point the water in an ice cold lake 4 miles 

 long, 4 miles wide, and 40 fathoms deep! 



Source of the Carbon Dioxide. — Such figures give one some 

 conception of the enormous activity associated with photosyn- 

 thesis, and when it is recalled how much carbon dioxide is taken 

 from the air annually by all the plants in the world, it is little 

 wonder that people refused to accept for a long time the con- 

 clusions of the early investigators. Where does all this carbon 

 dioxide come from? What are the sources which keep this sea 

 of carbon dioxide from being pumped dry by the photosynthetic 

 action of the green vegetation? 



First in quantity, but much more recent historically, is the 

 carbon dioxide resulting from the combustion of wood, coal, gaso- 

 line, etc. When these products are burned, the carbon again 

 combines with oxygen and returns to the atmosphere as carbon 

 dioxide. Over 500,000,000 tons of coal are burned annually, which 

 makes more than twice that much carbon dioxide. Secondly in 

 importance is the carbon dioxide resulting from the respiration 

 of plants and animals. In these oxidation processes within the 

 body, the carbon and oxygen are united and given back to the 

 air. An adult man gives off about 900 grams of carbon dioxide 

 daily. A third important source is the decay of organisms, during 

 which process carbon dioxide is given off to the air. Rocks, par- 

 ticularly limestones, contain large amounts of carbonates, which 

 give off carbon dioxide to the air during the processes of weather- 

 ing and disintegration. Finally volcanic action should be men- 

 tioned, since the gases given off during eruptions are rich in car- 

 bon dioxide. 



Distribution of Carbon Dioxide. — All these various sources 

 tend to restore the carbon dioxide taken from the air and to keep 

 the amount present about the same. The distribution over the 

 surface of the earth, owing to diffusion and the action of wind 

 currents, is everywhere nearly equal, although, because of the 

 fact that carbon dioxide is heavier than the nitrogen and oxygen 

 which make up the bulk of the atmosphere, the air near the earth 

 tends to be slightly richer in carbon dioxide than that at high 



