38 THE GEOGRAPHY OF PLANTS. 



purposes of vegetable life. A process, similar 

 to that of respiration in animals, is continually 

 going on in plants, but with a remarkable 

 difference. Carbonic acid is being continually 

 absorbed both by the roots and leaves of plants, 

 especially during the day ; while, under the 

 influence of light and heat, this absorbed acid 

 is decomposed by the vessels of the plant, the 

 carbon being taken up and assimilated as 

 nutriment, and the oxygen exhaled and re- 

 turned to the air ; but, during the night, car- 

 bonic acid is given off unaltered in its nature, 

 light being absolutely necessary for its decom- 

 position. This will explain and corroborate 

 the common notion that plants in a sleeping- 

 room are unhealthy at night, as the carbonic 

 acid, which is given off, is injurious. It will 

 easily be seen, also, that in the tropics, where 

 there is a clear, unclouded sky, and where light 

 and heat are most intense, oxygen will be 

 thrown off during the day in the greatest 

 abundance. This explains the greater luxuri- 

 ance of vegetation in those regions, the stimu- 

 lating influence of these agents causing a much 

 larger amount of nutriment to be appropriated. 

 On the contrary, in the higher latitudes, where 

 light is feebler and the vegetation less vigorous, 

 carbonic acid is given out in surplus quantity. 

 The currents of the air, however, and the 

 winds, restore the equilibrium, the oxygen of 

 the tropics being carried towards the poles. 

 and the carbonic acids of the colder climate in 

 the direction of the equator, to feed the thick 



