HOW PLANTS EAT. 35 



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 for- 

 get this most important 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 with 

 transparent walls, and no colouring matter (Fig. 

 i). These cells are full of water; they form a 

 sort of water-cushion on the top of the leaf, which 

 drinks in carbonic acid (or, to be quite correct, its 

 floating form, carbon dioxide) from the air about 

 it. Immediately below this cushion of water-cells 

 you come again upon a firm layer of closely-packed 

 green cells, filled with living green-stuff, which take 

 the carbonic acid in turn from the water-cells, and 

 manufacture it forthwith into sugars, starches, and 



