NATURE OF PLANTS 15 



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 which is the living substance containing the chlorophyll, 

 uses the energy of the light to bring about the recombination of 

 CO2 and H2O 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 photosynthe- 

 sis. 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 CO2 in the air, of which it forms a very small part, only 

 about three parts in ten thousand. Furthermore the carbon 

 comprises only 3/1 1 by weight of the CO2, 8/1 1 being oxygen. 

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

 CO2 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 CO2 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 1909 built up over 

 6700 millions of pounds of cotton, 725 million bushels of wheat, 

 984 million bushels of oats, etc., representing a value for all crops 

 of 8^ billion dollars. Our leading crop is corn which amounted 

 to over i^ billion bushels and valued at about 720 million dollars. 

 The gold and silver coin and bullion of the country are not of 

 greater value. This corn came up and matured in 120 days, 



