CHAPTER I. 

 THE AIR. 



All plants and animals pass their lives at the bottom of the great ocean 

 of air that surrounds the earth. From this air plants derive by far the 

 greater part of their substance. Pure air is a mixture of two invisible gases, 

 nitrogen and oxygen. The nitrogen is for the purpose of diluting the oxygen 

 so that animals can breathe it. But the two gases are not combined with 

 each other. The nitrogen is called free nitrogen because it is perfectly free 

 from any combination with other elements, but in order that plants can use 

 it it must become so combined. But of this later. We have said that plants 

 derive the greater part of their substance from the air. In fact about 95 

 per cent, of every plant comes from the air, or to speak more correctly, from 

 matters that exist as an impurity in the air. For while pure air is composed 

 of the two gases mentioned, there are always more or less of other gases in it, 

 not as essential parts of the air for animal life, but as impurities and detri- 

 mental to animals. But with plants the case is very different. One of the 

 impurities in the air is the gas called, commonly, carbonic acid, and which 

 is known to the chemist as carbon-di-oxide ; that is, it consists of two parts 

 of oxygen associated and combined with one part of carbon. This carbon- 

 di-oxide is the gas that accumulates in old wells and mines and makes what 

 is called choke damp, so deadly to animal life. But there is always a minute 

 portion of it in the air everywhere, and while an excess is damaging to animal 

 life a small portion is essential to the welfare of all plants that make green 

 leaves. 



HOW GREEN LEAVED PLANTS GET FOOD FROM THE AIR. 



All growth in plants and animals alike, is made by the increase in number 

 and the development of certain microscopic boxes known as cells. A very 

 thin cross section of a piece of the pith of the elder will show the primary 



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