NATURE OF PLANTS 15 



seven colors, i. e., red, orange, yellow, green, blue, indigo and 

 violet. When, however, a beam first passes through an alcoholic 

 solution of chlorophyll or through a green leaf a portion of certain 

 colors and all of other colors do not appear. The chlorophyll has 

 taken up or absorbed certain of the light rays especially portions 

 of the red, blue and violet. It is reasonable to suppose that the 

 energy in these rays is utilized in part by the plastids in effecting 

 the changes noted above in the formation of foods. The sun 

 gives to the earth great quantities of energy in the form of light. 

 A portion of this light is taken up by the chlorophyll, and the 

 plastid uses the energy of the light to bring about the recombi- 

 nation of CO 2 and H 2 O and the formation of sugars, starches, 

 and other carbohydrates. Because of the importance of the 

 sunlight in the formation of carbohydrates this process is called 

 photosynthesis. This term means the putting together by means 

 of light. 



7. The Magnitude of the Work of Photosynthesis. Carbon is 

 an important element in the composition of plant foods and also 

 in the walls of the cells. It forms one half of the dry weight of 

 the plant. All the carbon appearing in the plant is derived solely 

 from the CO 2 in the air, of which it forms a very small part, only 

 about three parts in ten thousand. Furthermore the carbon 

 comprises only 3/11 by weight of the CO 2 , 8/n being oxygen. 

 A square meter of leaf surface, however, can withdraw all the 

 CO 2 from 2500 liters of air in one hour. Such a volume of air 

 would be a space one meter square and 2% meters high. This 

 would furnish sufficient carbon for the construction of one gram 

 of starch. In this way such large quantities of CO 2 are drawn 

 into the chlorenchyma cells as to make possible each year the 

 harvest and the renewal of vegetation of the earth. In the 

 United States alone in this way there was in 1913 built up over 

 6500 millions of pounds of cotton, 763 million bushels of wheat, 

 1 121 million bushels of oats, etc., representing a value for all 

 crops of 6 billion dollars. Our leading crop is corn which amounted 

 to nearly 2% billion bushels and valued at about i l /2 billion 

 dollars. The gold and silver coin and bullion of the country are 

 not of greater value. This corn came up and matured in 120 



