HOW PLANTS GROW. 595 



that the under side of one leaf will often have many thousands 

 of them, and to show how fast they let out moisture from the 

 plant, cut a branch off full of green leaves, and see how quickly 

 they wilt and dry. But the most important thing these mouths 

 do, in common with other sorts of mouths, is to get food for the 

 plant from the air. The air, we have seen, is a fluid, made of 

 two gases, oxygen and nitrogen. Mixed with this pure air we 

 often find other gases, one of which, called carbon dioxide, is 

 made of two parts of oxygen gas, combined with one part of 

 carbon. This gas is heavier than pure air, and always settles to 

 the ground, and is often found in deep wells, when it is called 

 bad air, and suffocates those who go into the well. But this 

 gas, so poisonous to animals, is food for plants. So, while the 

 sun is shining, and at no other time, the little mouths in the 

 leaves open and suck in, as it were, this carbon dioxide from 

 the air. It passes through the vacant spaces between the little 

 cells, where the leaf-green is, and is brought in contact with it. 

 The leaf-green has the power of selecting the carbon, which the 

 plant wants, from the oxygen, and holds on to it, but leaving 

 most of the oxygen free to return to the air, thus rendering it 

 more pure for animals to breathe. Thus, with part of the 

 oxygen, the carbon, and the water taken up by the roots, the 

 leaf-green makes starch. After the sun has quit shining, and 

 night has come on, the plant takes the starch made during the 

 day, and changes it into other things, to build up new cells, 

 and make more stems, leaves, and roots. The little honeycomb 

 cells are made of stuff just like white paper, and it is called 

 cellulose. This cellulose is made of just the same thing that 

 starch is made of, but arranged by the plant so as to seem very 

 different. Then, with things the roots get from the soil, an<j 

 other things made from the starch, the little cells are gradually 

 filled up, and solid hard wood is made. We see, then, how the 

 warm sunshine helps the leaf-green make the starch from which 

 wood is made, and we can then realize that, when we warm our- 

 selves by burning wood, -we are only getting back the sunshine 

 which helped to make the wood long years before, and are 

 making new carbon di-oxide in the smoke, to go out into the air 

 and help to make new trees ; and when we see the small pile of 

 ashes left from the largest stick of wood, we realize how much 



