THE MANUFACTURE OF FOOD BY LEAVES 253 



and second, the various factors which modify the rate of photo- 

 synthesis. 



As the student already knows, carbon dioxide and water 

 furnish the elements from which sugar is synthesized. The 

 carbon dioxide is obtained from the air through the stomata, 

 while the water is brought up from the roots through the vascular 

 system, which through its numerous fine divisions in the meso- 

 phyll supplies either directly or indirectly all of the chlorenchyma 

 cells. The carbon dioxide is dissolved in the water with which 

 it passes into the cells and comes in contact with the chloroplasts 

 where the photosynthetic process takes place. The details of 

 the process involved in forming sugar from carbon dioxide and 

 water are not well known; but, leaving out the intermediate steps, 

 the equation 6C0 2 + 6H 2 O = C6H 12 6 + 60 2 represents the 

 nature of the process. From the equation it is seen that there 

 are as many molecules of oxygen liberated as molecules of carbon 

 dioxide used. Whether all or only a part of the 6O 2 liberated 

 for each molecule of sugar formed comes from the carbon dioxide 

 is not known. It is possible that only the carbon of the carbon 

 dioxide is used, in which case all of the oxygen liberated comes 

 from the carbon dioxide, or it may be that both water and carbon 

 dioxide have their constituents dissociated and some oxygen from 

 each is included in the 60 2 . 



Since photosynthesis removes carbon dioxide from the air to 

 which it returns an equal amount of oxygen, it is obvious that 

 it purifies the air and makes it more wholesome for animal life; 

 for animals in their respiration use oxygen and liberate carbon 

 dioxide, which, if allowed to accumulate, becomes injurious to 

 animals. Not only ordinary respiration of both plants and 

 animals but also fermentation, ordinary combustion, and all 

 other processes which use oxygen and liberate carbon dioxide 

 have their effects on the air counteracted by photosynthesis. On 

 the other hand, the oxidation processes maintain the supply of 

 carbon dioxide for photosynthesis. Thus photosynthesis and 

 the oxidation processes tend to support each other. 



Photosynthesis takes place in the chloroplast, but the exact 

 function of either the body or the chlorophyll of the chloroplast 

 is not known. It is generally believed that the chief function of 

 the chlorophyll is to provide energy for the process; and this it 

 does by transforming the sun's rays into available forms of 



