VEGETABLES. 85 



GROWING PLANTS PURIFY THS AIR. 



143. Wheu a plant has put forth its first leaves, and 

 is no longer dependent on the seed for jts support, it 

 reverses the process before described — absorbs car- 

 bonic acid and emits oxygen, during the day and so 

 long as light continues, but still absorbs oxygen and 

 emits carbonic acid in the night. The carbonic acid is 

 decomposed in the plant, and its carbon wrought into 

 the solid texture of the plant, while its oxygen is given 

 off. Other floating gases are taken into the soil and 

 conveyed to the plant through its roots. Thus growing 

 plants purify the air of those gases which render it 

 unhealthy for respiration ; while the respiration of 

 men and beasts enriches it with those gases which 

 promote vegetation ; so that plants and animals are 

 mutually beneficial, each rendering the air health-giv- 

 ing to the other. None breathe so invigorating an at- 

 mosphere, as the farmer among his growing crops. 



SOURCES OF CARBON AND OTHER FOOD TO 

 PLANTS. 



144. During the growth of the plant it takes its 

 carbon mainly from the air. A little is believed by 

 physiologists to pass in through the roots, dissolved 

 in water. Its oxygen and hydrogen are undoubtedly 

 furnished mostly in the form of water, and in that 

 form taken in both by the roots and leaves. 



145. Nitrogen is furnished to plants principally in 

 the form of nitric acid and ammonia, both of which 

 exist in the air and in rain-water. 



