HOW PLANTS GET FOOD FROM SOIL AND AIR 35 



contains is there taken from it for use in the body. If 

 persons and land animals remain long under water they 

 drown, because the supply of air is cut off. Likewise 

 ordinary plants die when their leaves or roots are kept long 

 under water, because they are thus deprived of sufficient 

 air. 



How air enters the leaf. Why do you suppose that 

 the leaf is made so thin and broad instead of being rolled 

 up into a compact, round little bundle ? Leaves, even the 

 thinnest, are made up of several very thin layers, each one 

 of which consists of a great number of cells. In the outer 

 layers of the leaf these cells lie close together, making 

 a tight thin sheet which covers layers of more loosely 

 arranged inner cells (Fig. 28). In these thin outer layers 

 are great numbers of tiny openings. These are gateways 

 for the entrance of air into the inside of the leaf. They 

 are also the gates through which leaves get rid of the 

 surplus water sent up by the roots. They are too small to 

 be seen with the naked eye, and exist in large numbers 

 on every square inch of the under side of every leaf. The 

 plant can open and close these by means of special cells 

 called guard-cells. When the weather is very dry, these 

 gateways partly close, to keep moisture from passing too 

 rapidly out of the leaves. 



Use of air by plants. Plants and animals are alike in 

 "breathing" fresh air, containing oxygen. Man and ani- 

 mals take oxygen from the air to use in their lungs and 

 give back a gas called carbon dioxid, which consists of 

 carbon and oxygen united together. Plants, too, use oxy- 

 gen, although they have no lungs. In making food for 



