114 APPLIED BIOLOGY 



dioxide (CO 2 ) and water (H 2 O) . The water thus formed can- 

 not be distinguished from the other water which is abundant 

 in plant cells. The carbon dioxide is transported (probably 

 chiefly in solution in the moving liquids in plants), to the 

 surface, especially of the leaves, and then diffuses to the sur- 

 rounding air. 



As we have seen in 105, a plant with chlorophyll does 

 not appear to give off carbon dioxide in light, because the 

 starch-making machinery is using that gas much more rapidly 

 than the cells of the same plant are making it. If any carbon 

 dioxide made in the cells of the roots or of the stem is carried 

 in the water current to the leaves, it may be used in starch- 

 making; and in addition, the leaves must continually 

 absorb more of the gas from the air. As has been shown, a 

 closed jar containing a measured quantity of carbon dioxide 

 and a green plant, and placed in sunlight, will have less of 

 the gas after a few hours; while a similar jar and plant 

 kept in total darkness will have more carbon dioxide in 

 the air of the jar. In the first jar the plant must have used 

 for starch-making all the carbon dioxide produced by oxida- 

 tion from its own cells and in addition some of the gas 

 taken from the air. 



107. Other Excretions of Plants. The term excretions 

 is commonly applied to such substances as carbon dioxide, 

 which are produced by oxidation in the cells of plants and 

 animals, and which are eliminated because they are of no 

 further use in the cells, and are sometimes actually poisonous. 

 Carbon dioxide and water have been mentioned above as two 

 excretions formed by oxidation of cell-substances containing 

 the elements carbon and hydrogen. The water formed by 

 oxidation mixes with the water taken into the plant by the 

 roots, which is being eliminated continually by evaporation. 



In addition to carbon dioxide and water, plant cells form 

 other excretions and also have an excess of certain substances 

 containing the elements absorbed with water from the soil. In 



