24 OF THE ASSIMILATION OF CARBON. 



rays of the sun to shine upon an immeasurably 

 luxuriant vegetation. The temperate and cold 

 zones, where artificial warmth must replace deficient 

 heat of the sun, produce, on the contrary, carbonic 

 acid in superabundance, which is expended in the 

 nutrition of the tropical plants. The same stream 

 of air, which moves by the revolution of the earth 

 from the equator to the poles, brings to us, in its 

 passage from the equator, the oxygen generated 

 there, and carries away the carbonic acid formed 

 during our winter. 



The experiments of De Saussure have proved, that 

 the upper strata of the air contain more carbonic 

 acid than the lower, which are in contact with 

 plants ; and that the quantity is greater by night 

 than by day, when it undergoes decomposition. 



Plants thus improve the air, by the removal of 

 carbonic acid, and by the renewal of oxygen, which 

 is immediately applied to the use of man and 

 animals. The horizontal currents of the atmo- 

 sphere bring with them as much as they carry 

 away, and the interchange of air between the 

 upper and lower strata, which their difference of 

 temperature causes, is extremely trifling when 

 compared with the horizontal movements of the 

 winds. Vegetable culture heightens the healthy 

 state of a country, and a previously healthy country 

 would be rendered quite uninhabitable by the ces- 

 sation of all cultivation. 



The most important function in the life of plants, 



