CHAPTER XIX 



RESPIRATION AERATION FERMENTATION 

 Respiration. This term was first used and is still used in 

 reference to the breathing of animals. For a long time it was 

 supposed that plants breathed in very much the same manner 

 as animals, and therefore it was used to refer to the breathing of 

 plants. But we have already learned that plants do not breathe 

 in the same manner as animals. The air is not taken in and 

 forced out as in the animal, but there is an exchange of at- 

 mospheric gases by diffusion (see page 173). The term " res- 

 piration " more strictly refers to the catabolic process in the 

 individual cell, whether animal or plant. It occurs in all active 

 cells throughout the plant and is not restricted to the leaves. 

 It requires oxygen, part of which is obtained as a result of 

 photosynthesis and part of which is taken in through the leaves 

 in the same manner as the carbon dioxide (see page 195) and 

 then transferred to other parts of the plant in solution. It may 

 also obtain oxygen as a result of the metabolic activities of the 

 cell. When the carbon of the starches, sugars and other com- 

 pounds of the cell unites with the oxygen, heat is liberated, but 

 most of it is again utilized by the plant. This process can be 

 illustrated by the following formula : 



C 6 H 12 O 6 -f 6O 2 = 6CO 2 + 6H 2 O 



(Grape sugar) (Oxygen) (Carbon (Water) 

 dioxide) 



The heat causes the temperature of the plant to be a little higher 

 than the surrounding atmosphere. The by-products of respira- 



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