60 ELEMENTS OF AGRICULTURE 



CHAPTER XI.— The Food the Plant Takes from 

 the Air 



59. Carbon. — Plants, as yon have seen, get their water 

 and mineral matter from the soil. You would then 

 naturally inquire whether plants get their carhon from 

 the soil. Carbon in some form is present in all soils, but 

 plants do not use this- carbon for food. It has been 

 proven by experiments that plants cannot grow unless 

 their leaves are supplied with air containing carbon 

 dioxide gas, which you remember makes up a small part 

 of the atmosphere. Other experiments have shown that 

 the leaves of plants absorb carbon dioxide from the 

 air, take from the gas the carbon, and give off the 

 oxygen. Men and animals reverse this process; they 

 draw air into their lungs, where the oxygen of the air 

 unites with the carbon in the blood, and is given off as 

 carbon dioxide gas. Men and. animals inhale oxygen 

 and exhale carbon dioxide. Plants inhale carbon dioxide 

 and -give off oxygen. Animals cannot live in air con- 

 taining much carbon dioxide. Plants cannot live with- 

 out a supply of this gas. Leaves have often been called 

 the lungs of plants, and in their manner of taking up 

 food from the air they do resemble the lungs of animals. 

 Breathing animals are constantly adding to the supply 

 of carbon dioxide in the air ; the leaves of plants are con- 

 stantly absorbing it and returning to the air free oxy- 

 gen. In this way plants aid in maintaining a balance, 



