NATURE OF PLANTS 15 



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 decomposition of 

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

 three or four 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 1000 liters of air in one hour. 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 chlor- 

 enchyma 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 1906 built up over 2700 millions of 

 bushels of corn, 770 million bushels of wheat, 850 million bushels 

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

 dollars. These figures represent only a small portion of the work 

 performed, since the grain is but a part of the plant, and further- 

 more the larger portion of the land is covered with forests and 

 other forms of vegetation. The fact must not be overlooked that 

 this process of photosynthesis is of vital importance to our welfare 

 in another way. Owing to the large volumes of CO 2 that are 

 constantly formed by fires and the respiration of animals such an 

 excess of this gas would accumulate in the atmosphere that all 

 animal life would soon cease were it not for its absorption in 



