KESPIRA TWX. 



117 



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

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

 ofCO^. Green plants, i.e., plants with chlorophyll, purify the air during 

 photosynthesis by absorbing COg and giving off oxygen. Animals absorb 

 in respiration large quantities of oxygen and exhale large quantities of CO2 

 Plants absorl> a comparatively small amount of oxygen in respiration and 

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

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

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

 percentage of CO^ in the air remains approximately the same, viz., al>.>i!t 

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

 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 of 

 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 

 possess chloroph^dl 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 fr.rmation or 

 Photosynthesis. 



Respiration. 



