188 THE NATURE AND yVOBK OF PLANTS 



million pounds. Fifty square yards of leaf surface 

 will take up as much carbon dioxide as may be 

 thrown off by a single person, and furnish as much 

 oxygen as he would need. When the immense area 

 of the leaves of all the plants in the world is con- 

 sidered, it is found at the extremest low estimate 

 that the vegetable world uses more than twice as 

 much carbon dioxide as may be produced by living 

 agencies, and gives off twice as much oxygen as 

 they consume. No appreciable change has ever 

 been found in the composition of the atmosphere 

 with respect to these gases, however, and it must 

 be concluded that other agencies are at work which 

 use these gases ; otherwise the air would be grow- 

 ing poorer in carbon dioxide and richer in oxygen. 

 It is found that changes are constantly going on 

 in the soil and in the rocks, which liberate and 

 take up these gases in quantities which make the 

 amounts used by living things seem very insignificant, 

 and that the waters of the sea form a vast store- 

 house for them. Then again the atmosphere con- 

 tains about eight thousand billions of pounds of 

 carbon dioxide, and if the activities of plants were 

 to cease entirely, it would be many hundreds of 

 years before the proportion of gases would be 



