RELATION OF CARBON DIOXIDE TO PLANT GROWTH 193 



dioxide are taken up by the plant, but the carbon dioxide being 

 about thirty times more soluble than the oxygen is taken up 

 much more rapidly. Both of these gases are also given off by the 

 plant, but the oxygen is given off in far greater abundance. 

 Since the animal takes in oxygen and gives off carbon dioxide, 

 it will be readily seen that plants and animals are m.utually help'- 

 ful to each other. 



Absorption of Carbon Dioxide. — Gases of the atmospheire 

 pass by diffusion in and out of the stomata (see page 17-2) and 

 thus come in contact with the very delicate walls of the inner 

 cells of the leaves and other green parts of the plants. These cell 

 walls are always moist with imbibed water, Therefore, since 

 we have a liquid on both sides of the cell walls we have the neces- 

 sary conditions for liquid transfusion and for osmosis. More 

 or less CO2 is absorbed by the water on the outside of the cell 

 wall which becomes carbonic acid. Oxygen from the atmosphere 

 is also absorbed, but, as previously stated, the carbon dioxide 

 is thirty times more soluble than oxygen and, therefore, is ab- 

 sorbed much more rapidly. More or less of this weak carbonic 

 acid passes through the cell wall and thus into the cell sap, where 

 it is acted upon by the sunlight and used in the formation of car- 

 bohydrates, as, will be described later (see page 195). 



The Cells, the Factories of the Plant. — ^We have already 

 learned that the living cells contain protoplasm, which is 

 necessary for the formation of all other plant products. We 

 have also learned that certain of the plant cells contain chloro- 

 phyll which is necessary for the making of carbohydrates. If 

 the plant did not contain chlorophyll, it could not manufacture 

 carbohydrates and life would be impossible. Therefore, chloro- 

 phyll is one of the most important substances in nature. The 

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