The Food of Plants 19 



there is in the air for about a mile in height above the How much 

 acre. We can see why so much carbon dioxid must 



be taken in by plants when we realize that charcoal, a cr P 

 which is the part that turns black when any plant sub- 

 stance is partially burned, consists almost purely of 

 carbon. This carbon, of course, represents the carbon 

 dioxid that the plant drew from the air through the 

 stomata, the little openings in Jjie leaves. 



Carbon dioxid, so necessary to plant life, is interesting Common 

 also because of the many uses to which it is put. In ^mbon 

 the manufacture of soda water and all other carbonated dloxid 

 waters, it is dissolved in the liquid under pressure. 

 When the pressure is removed by drawing a cork or 

 turning a faucet, the gas bubbles up, causing the effer- 

 vescence. This same gas is sometimes used in the 

 manufacture of ice. It is easily compressed to liquid 

 form, and when the liquefied gas evaporates and expands, 

 it draws heat away from the water, and so freezes it. 

 (Exp. n.) Bread is made to rise by carbon dioxid. 

 The gas is generated all through the dough by the 

 baking powder or yeast, and the bread is kept expanded 

 until it is baked so that it cannot " fall " when the gas 

 escapes. 



The plant life of the world daily uses thousands of How 

 tons of carbon-dioxid gas, and if this did not get back Dioxid is 



into the air the supply would soon be exhausted and lost from 



^ J the air 



nothing could grow. There are three ways in which 



the gas is set free from the plant substance in which 



it has been imprisoned. Plants either decay, are burned 



up, or are eaten. When they decay the carbon changes How it is 



slowly to carbon dioxid gas, and goes back into the air. 



