III. AIR AND SUNLIGHT 



The Air. The earth is surrounded by a mixture of several 

 gases which we usually call the air or atmosphere. These gases and 

 their average amount by volume are as follows: 



Nitrogen ' 77.50 parts 



Oxygen 20.64 parts 



Argon 1.00 parts 



Carbon Dioxide 04 parts 



Water Vapor 82 parts 



Total .... 100.00 parts 



Besides these there are traces of other constituents, such as 

 ammonia, nitric acid, dust, ozone, and a few other substances. 

 About 5 per cent of the food of the plant is mineral matter from 

 the soil and the other 95 per cent is made up of water (composed 

 of hydrogen and oxygen), carbon, nitrogen, and oxygen, all of 

 which are contained in air. 



Carbon. Our most familiar example of carbon in a solid form 

 is found in mineral coal and charcoal. If we partly burn a pine 

 splinter and note carefully the black substance that is formed, we 

 find that it is carbon. When wood or coal is burned, the carbon 

 in these substances unites with the oxygen in the air and forms an 

 invisible gas called carbon dioxide. 



All the carbon in the plant comes from the air, and while the 

 amount of carbon dioxide present in the air seems relatively small, 

 there is more than enough present to supply all the demands of 

 vegetation. Professor Storer estimates that there is enough of 

 this gas to approximate twenty-eight tons for every acre of the 

 earth's surface. According to Professor Chevandier an acre of 

 thrifty beech trees will assimilate about three tons of carbon dioxide 

 in a year ; and if the whole earth were covered with a forest of such 

 trees, it would require more than nine years to consume all this gas 



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