HOW PLANTS EAT. 37 



into the atmosphere ; the carbon, uniting with 

 oxygen, goes off in the form of carbonic acid, 

 while the hydrogen, uniting with oxygen, goes off 

 in the form of steam or vapour of water. What 

 is there left ? A very small quantity of solid 

 matter, which we know as ash. Well, that ash, 

 which returns to the soil in the solid condition, 

 is practically almost the only part the plant got 

 from the soil ; the rest returns as gas and vapour 

 to the air and water, from which the plant took 

 them. You must never forget this most im- 

 portant fact, that plants grow mainly from air 

 and water, and hardly at all from the soil beneath 

 them. Unless you keep it firmly in mind, 

 you will not understand a great deal that 

 follows. 



Why, then, do gardeners and farmers think so 

 much about the soil and so little about the air, 

 which is the chief source of all living material ? 

 We shall answer that question in the next 

 chapter, when we come to consider What Plants 

 Drink, and what food they take up dissolved in 

 their water. 



Carbonic acid, though itself a gas, is the chief 

 source of the solid material of plants. How do 

 plants eat it ? By means of the green leaves, 

 which suck in floating particles of the gaseous 

 food. Their eating is thus more like breathing 

 than ours : nevertheless, it is true feeding : it is 

 their way of taking in fresh material for building 

 up their bodies. If you examine a thin slice 

 from a leaf under the microscope, you will find 

 that its upper surface consists of a layer of cells 



