RES P IK A TION. 



117 



239. Importance of green plants in maintaining purity of air. By respi- 

 ration, especially of animals, the air tends to become " foul " by the increase 

 of CO 2 . Green plants, i.e., plants with chlorophyll, purify the air during 

 photosynthesis by absorbing CO 2 and giving off oxygen. Animals absorb 

 in respiration large quantities of oxygen and exhale large quantities of CO 2 

 Plants absorb a comparatively small amount of oxygen in respiration and 

 give off a comparatively small amount of CO 2 . But they absorb during 

 photosynthesis large quantities of CO 2 and give off large quantities of oxygen. 

 In this way a balance is maintained between the two processes, so that the 

 percentage ofCO 2 in the air remains approximately the same, viz., about 

 four- tenths of one per cent, while there are approximately 21 parts oxygen 

 and 79 parts nitrogen 



239a. Comparison of respiration and photosynthesis. 



Carbon dioxide is taken in by the plant and oxygen 

 is liberated. 



Starch is formed as a result of the metabolism, or 

 chemical change. 



The process takes place only in green plants, and in 

 the green parts of plants, that is, in the presence 

 of the chlorophyll. (Exception in purple bacte- 

 rium. ) 



The process only takes place under the influence oi 

 sunlight. 



It is a building-up process, because new plant sub- 

 stance is formed. 



Oxygen is taken in by the plant and carbon dioxide 

 is liberated. 



Carbon dioxide is formed as a result of the meta- 

 bolism, or chemical change. 



The process takes place in all plants whether they 



Respiration. \ 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 disintegra- 

 tion of plant substance occurs. 



Starch formation or 

 Photosynthesis. 



