Leaves and their Work 169 



crease in size by the multiplication of cells, it is evident 

 that they could not grow at all in air containing no 

 carbon-dioxide. Neither can they grow if deprived of 

 their leaves, for these are the chief manufacturers of 

 cellulose and other food. A tree stripped of even half 

 its leaves will be unable to make much wood, though it 

 may manage to live. 



The skin of a leaf usually consists of a single layer of 

 cells, not green, but colourless and transparent, and 

 beneath these are other cells containing, besides other 

 things, * leaf-green,' or colouring matter. It is in these 

 lower cells that the manufacture of the plant's food is 

 carried on ; and, though the process cannot be ex- 

 plained, one or two facts are certain it cannot, in 

 most plants, go on without light, or in any without the 

 leaf-green. 



The gases of the air are able to pass through the 

 cell-walls, both in and out. It must not be forgotten 

 that plants need air for breathing, as well as carbon- 

 dioxide for food ; and though they breathe as well as 

 feed by means of their leaves, the two processes are 

 quite distinct. 



What the plant does with the carbon-dioxide is to 

 separate the carbon and keep it, and to let go most of 

 the oxygen. The two have to be torn asunder, and 

 this is done in the cells containing leaf-green. But the 

 leaf-green itself cannot be developed either without 

 light, or without iron ; and when developed it cannot 

 act in darkness. 



For the supply of iron the leaves are, of course, 

 dependent upon the plant's roots, and if the roots can- 

 not find it, the leaves and young stems remain yellow 

 or colourless. Compounds of iron are, however, so 



