RESPIRA TION 



123 



Respiration. 



Oxygen is taken in by the plant and carbon 



dioxide is liberated. 

 Carbon dioxide is formed as a result of the 



metabolism, or chemical change. 

 The process takes place in all plants whether 



they possess chlorophyll or not (exceptions 



in anaerobic bacteria). 

 The process takes place in the dark as well 



as in the sunlight. 



It is a breaking down process, because disin- 

 tegration of plant substance occurs and 



energy is released. 



198. Green plants maintain the purity of the air. Res- 

 piration, especially of animals,* increases the carbon dioxide of 

 the air, and if there were not some process in nature to counteract 

 this, the air would become "foul" and animal life would cease 

 to exist. In close rooms where there are many people, the air 

 soon becomes "foul" because of the decrease of oxygen and 

 the increase of carbon dioxide. Green plants by the process of 

 photosynthesis purify the air. They absorb carbon dioxide and 

 give off oxygen, thus maintaining over the face of the earth the 

 ordinary proportions of oxygen and carbon dioxide. In the 

 water inhabited by plants and animals a similar equilibrium of 

 oxygen and carbonic acid is maintained. This is why it is 

 usually necessary to have some aquatic plants in an aquarium 

 with aquatic animals. 



* Comparatively little carbon dioxide is given to the air by plant respira- 

 tion, since, during the day, that formed in the green parts of plants is used 

 in photosynthesis. 



