CHAPTER XVII 



USES OF FOOD IN PLANTS 



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I. RESPIRATION 



In the chapter on photosynthesis attention was called to the early experi- 

 ments from which it was concluded that carbon dioxide increases and 

 oxygen decreases in the air surrounding all parts of plants in the dark, 

 and non-green parts of the plant in both light and dark. The results of 

 numerous later experiments showed this conclusion to be true both for 

 green plants and for many non-green plants. They also disclosed the 

 kinds of exceptions to this conclusion that may be found among the non- 

 green plants. 



One may easily demonstrate some of these facts by enclosing a mass 

 of germinating seeds, opening flower buds, green leaves, or any other 

 actively growing parts of a plant in tightly stoppered bottles for several 

 hours, and then testing the enclosed air for carbon dioxide and oxygen. 

 When the air from one of these bottles is forced into limewater an 

 abundance of carbon dioxide is indicated by the rapid formation of a 

 precipitate of calcium carbonate. The decreased amount of oxygen in 

 another bottle may be detected by chemical tests, or by the failure of 

 an ignited match to continue burning in it. By pressing a finger into the 

 mass of seeds in one of the bottles one becomes aware of the higher 

 temperature of the germinating seeds. A smaller increase in temperature 

 may also be detected in the bottles containing opening flower buds and 

 leaves. This increase in temperature may be measured if the plant parts 

 are kept for a few hours in a thermos bottle into which a thermometer 

 extends. 



All these changes in amounts of carbon dioxide and oxygen, and the 

 increase in temperature in the demonstrations are dependent upon the 

 activity of living plant cells. They do not occur in similar demonstrations 

 containing only sterilized dead plants or dead parts of plants. 



These demonstrations may also be repeated with germinating seeds 

 that have been sterilized and placed in bottles containing air from 

 which all the oxygen has been removed. In the absence of free oxygen, 



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