CHAP. II.] CHEMISTRY, ETC., OF PLANT LIFE. 71 



functions will make clear what at first might be con- 

 sidered as a contradictory statement. 



In the act of nutrition, as already explained, carbonic 

 dioxide is removed from the atmosphere in considerable 

 quantities and used by the plant as food. This function 

 can only be exercised when the plant is exposed to light, 

 and ceases when the plant is placed in darkness ; conse- 

 quently, in a state of nature, plants only remove carbonic 

 dioxide from the atmosphere during the day. This act 

 is accompanied by the restoration of a considerable 

 amount of free oxygen to the atmosphere, in fact, an 

 equal amount to that taken in chemical combination 

 with the carbon as carbonic dioxide. The function of 

 respiration, which is common to all forms of life, both 

 animal and vegetable, is a purifying process, exercised 

 for the purpose of removing certain worn-out material, 

 especially carbon, from the body ; this substance is re- 

 moved in the gaseous form as carbonic dioxide. Oxygen 

 is inhaled from the atmosphere, and, chemically com- 

 bining in the tissues with the waste carbon, is exhaled 

 or returned to the atmosphere as carbonic dioxide. 

 This exchange of gases in the plant world is mainly 

 due to dififusion, the mechanical auxiliaries present in 

 connection with respiration in the higher animals being 

 absent from plants. The function of respiration, in 

 plants as in animals, is unaffected by the presence or 

 absence of light, and is exercised uninterruptedly during 

 the life of the individual ; hence in the performance of 

 this function plants are always removing oxygen from 

 and adding carbonic dioxide to the atmosphere; but 

 the respiration of plants is a comparatively slow process, 

 and as very much more carbonic dioxide is removed 



