THE TREE 17 



served the bark. This is in two layers, an outer protective 

 corky layer and an inner, light colored, softer layer sometimes 

 called the bast. Inside the bark is a narrow, light colored por- 

 tion of wood called the sapwood, and inside this extending to 

 the center of the tree, a darker portion called the heartwood. 

 In the center 'of the tree in some species will be found the 

 pith. Running from the bark toward the center of the tree 

 narrow lines of woody tissue will be seen, some extending even 

 to the pith. These are the pith rays or medullary rays. In 

 some trees like the oak, beech and sycamore the pith rays are 

 very conspicuous. They give the silver grain to a board 

 when quarter sawed. They carry food from the bark to the 

 inner living portions of the tree, store reserve food material 

 and form a means of communication with the air. They are 

 present in great numbers in all kinds of wood, thousands oc- 

 curing to the square inch, though often so small as to be in- 

 visible without a strong magnifying glass. In some species 

 as in birch and cherry their terminations in the bark are often 

 conspicuously marked by rough spots called lenticles. 



Between the bark and the sapwood is a layer of active living 

 cells, capable of division into other cells. This is called the 

 cambium layer. It is by the division of the cells in the cambium 

 layer that the trunk of a tree grows in diameter and the 

 branches and roots become larger. When first formed from 

 the cambium layer the cells are filled with protoplasm and re- 

 main alive as long as they are in the sapwood. In young trees 

 all the wood is sapwood. In time, varying in length with 

 different species, this sapwood begins to change to heartwood. 

 The cells lose the living protoplasm, become darker in color, 

 and the cell walls harder. They no longer conduct food 

 material to the crown. Heartwood is dead wood. It serves 

 no purpose in the life of the tree except to give strength and 

 rigidity to the stem. There is no definite age at which sap- 

 wood changes to heartw r ood. It is a more or less irregular 

 process. 



