Leaves and Their Work 141 



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 colorless and transparent, and beneath 

 these are other cells containing, besides other things, 

 '* leaf-green," or coloring matter. It is in these lower 

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

 on; and though the process cannot be explained, 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 sepa- 

 rate 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, de- 

 pendent upon the plant's roots, and if the roots cannot 

 find it, the leaves and young stems remain yellow or 

 colorless. Compounds of iron are, however, so very 

 general in all the rocks composing the earth's crust, that 

 it is almost impossible to find any soil quite without them. 



But the iron may be taken away by artificial means, 

 and when this is done the leaf-green turns yellow, as 

 human beings do when their blood contains too few red 



