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CHAPTER VII. 



THE FOOD OF PLANTS — ITS NATURE AND SOURCES. 



636. Having shortly described the elements of 

 vegetable matter, and enumerated the principal com- 

 pounds of those elements which are found in plants, 

 we may at once proceed to consider the sources of 

 the food of plants ; that is to say, the means natu- 

 rally provided to insure them a due supply of the 

 various substances necessary to their growth; namely, 

 oxygen, hydrogen, carbon, nitrogen, and the various 

 earthy and saline substances which are always found 

 in plants. 



637. There are only two sources whence it is pos- 

 sible for plants to derive these matters; namely, the 

 air and the soil ; let us inquire what substances they 

 can obtain in this manner, and how they avail them- 

 selves of the food thus offered to them. 



638. It has been already stated, that the air at 

 all times contains a small quantity of carbonic acid 

 gas (37) ; it likewise always contains a still more 

 minute proportion of ammonia, which is constantly 

 being formed by decay. Here, then, we see that the 

 air contains the four elements of organic matter; and 



