24 THE TREE BOOK 



the bark. But between wood and bark, from 

 root tip to twig end is the cambium, through 

 which, you remember, the sap avenues descend. 

 This is the important factor in the life of the 

 tree proper — the carpenter, if you please. It 

 builds new root and bark, lengthens branches 

 and roots, and adds to the girth of the tree, 

 using the material supplied by the leaves. 



The cambium is made up of millions of cells. 

 Always the cells have a disposition to increase 

 in size and divide in two parts. This division 

 goes on so long as the food supply lasts. Thus 

 one cell may increase to two, four, eight, sixteen, 

 thirty-two, sixty-four, etc. 



A new layer of woody tissue and one of bark 

 is formed by the cambium each year, adding to 

 the tree's thickness just that much. The cam- 

 bium never adds to its own width, but remains 

 always a thin layer of dividing cells, ambitious 

 only to add to the bark on one side and the wood 

 on the other. A tree grows taller only as the 

 terminal branch shoots upward. Cell division 

 is the most rapid in the green shoots, which 

 are practically made up of pith and cambium. 

 Being so full of chlorophyll, they can make their 

 own food, and hence are independent of the 

 leaves. Besides, the cambium cells here have a 

 wonderful power which is lacking in the cam- 



