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THE INDIVIDUAL ORGANISM 



requiring gaseous interchange with the environment. In the plant, on 

 the other hand, there are two distinct processes that must be separately 

 considered if confusion is to be avoided. 



Respiration goes on continuously in the cells of the plant, using oxygen 

 and producing carbon dioxide and water. At night, when photosynthesis 

 is inoperative, respiration is the only factor in the situation, and at this 

 time the leaf takes in oxygen and gives off carbon dioxide like an animal 

 body. With the coming of day, the photosynthetic manufacture of glucose 

 is resumed; but respiration also continues, if anything, at a faster rate 

 than during the night. Since, however, photosynthesis requires such 

 great quantities of carbon dioxide, the amount produced by respiration 

 of the cells, although utilized, is altogether insufficient, and carbon dioxide 

 from the outside begins to flow into the leaf. The cells continue to require 

 oxygen for their respiratory needs; but photosynthesis is producing it 

 in such great quantities as a waste product that there is an oversupply 

 within the leaf, and the excess that cannot be used in respiration diffuses 

 out through the stomata. 



It is often thought that because leaves take in carbon dioxide and give 

 off oxygen during the day, respiration in plants is exactly the reverse of 

 respiration in animals. In reality, it is identical in principle, and the day- 

 night reversal of intake and outgo of the two gases results from over- 

 balancing of respiration by photosynthesis during the day. The relations 

 may be seen in the following tabular comparison of the two processes: 



Synthesis of other materials from glucose. A part of the glucose 

 produced by photosynthesis is directly used by the leaves and other parts 

 of the plant, being oxidized with release of energy. A much larger por- 

 tion is chemically transformed into a variety of other substances useful 

 to the plant — food in forms suitable for storage; materials needed for 

 building protoplasm and for forming the cell walls; enzymes; and other 

 products. The majority of these syntheses are highly complex, and many 

 of them are not or are but poorly understood; but we can gain some 



