FOOD FROM THE SOIL AND FROM THE AIR. 21 



must be in a soluble state, capable of immediate 

 employment in building up the plant. The farmer, 

 then, who knows best what is needed, knows how to 

 furnish it so as to have the best crops, and at the least 

 expense. 



An examination of the leaves and of the roots of 

 a living plant, shows that it obtains a portion of its 

 food from the air, and a portion from the earth. 



a. Inorganic food, consisting as it does of solid 

 bodies, does not of course exist in the air, and must 

 therefore all be taken in through the roots. 



b. The organic food comes partly from the soil, and 

 partly from the air through the leaves. It may be 

 asked, How we know that plants get food through 

 their leaves? This is easily proved. If we place the 

 stem and leaves of a growing plant in a portion of 

 confined air, the composition of which is known, and 

 that air be reexamined by means of chemical tests a 

 day or two afterward, it will be found that its com- 

 position has changed: a portion of it has disappeared, 

 having been absorbed by the plant through its leaves. 



c. If the confined air, for instance, contained car- 

 bonic acid, a portion has gone, and its place is supplied 

 by oxygen. 



d. If there is no carbonic acid present in the water 

 or air, the action will not go on. The importance of 

 these facts will soon be perceived. 



We have seen something of the forms in which 

 plants may receive their inorganic food; that it is not 

 usually as simple substances, but in some forms of 

 combination. Thus potash does not enter the roots as 

 potash alone, but as sulphate or carbonate or silicate 

 of potash ; that is, in combination with sulphuric acid, 

 or carbonic acid, or silica. So it is with organic food; 

 the four gases which we have examined do not or- 

 dinarily minister in their simple state to the growth 

 of plants, but, as do the inorganic substances, in some 

 form of combination. 



