THE INTER-RELATIONS OF PLANTS AND MAN 113 



ment, the available supply of these two elements will eventually 

 become exhausted since no natural inorganic process produces 

 them in sufficient quantity to be of use. Millions upon millions of 

 tons of protoplasm exist in living bodies alone; to which must be 

 added the billions of tons which have gone into the bodies of past 

 plants and animals. 



Carbon dioxide is taken in during photosynthesis and from it 

 carbohydrate foods are manufactured. Without carbon, there 

 could be no food, and no life. The amount of carbon dioxide in 

 the world is definitely limited; the .03 per cent present in the 

 atmosphere is equivalent to about six tons over each acre of 

 earth's surface. Some crops such as sugar cane extract many 

 tons of carbon from the air above each acre in a growing season. 

 At the average plant consumption rate, it has been estimated that 

 all the carbon dioxide in the atmosphere would be used up in 

 thirty five years. Since life has been a going concern for many 

 millions of years, and no danger of a carbon shortage is present 

 today, there must be some way in which nature balances the 

 consumption and production of carbon dioxide. Respiration 

 returns small amounts to the air, but in such insignificant quanti- 

 ties that this would only defer the day when all the carbon dioxide 

 would be depleted. It is at this point that the bacteria and some 

 fungi enter the picture (fig. 76). There is a natural process known 

 as decay, which is considered by many to be a nuisance if not a 

 danger, since it causes the destruction of foods and other useful 

 articles containing organic substances. Decay is the result of the 

 activity of colorless plants, chiefly bacteria. These ubiquitous 

 organisms are found practically everywhere; and as soon as life 

 leaves a body, the bacteria start their work as scavengers and 

 decompose the remains. This is done so efficiently that we can 

 walk for miles through a forest in which life has gone on for 

 centuries, but find few traces of these organisms that have lived. 

 Decay bacteria, while living according to their saprophytic type 

 of metabolism upon the organic substances of the dead body, 

 thus become the world's number one scavengers. But they per- 

 form an even greater service; for during the process, carbohy- 

 drates and fats are broken down into the carbon dioxide and 

 water from which they were made. Wood, for example, is one of 



