The Growth of the Tree 33 



They are lines of living tissue running at right angles to 1 

 the other fibers and furnish a storehouse for the surplus 

 food saved over from one season to another. It is the 

 splitting of these rays that makes the silver grain in quarter- 

 sawed oak ; they are also important as the starting point 

 for checks in seasoned lumber. They are present in all 

 woods but are so small in many species that they cannot 

 be perceived without the use of a microscope. A further 

 variation may be noted in the appearance of the cross- 

 section. The outer portion, varying with the species, 

 from three to fifty rings in width, is of a lighter color than 

 the inner portion. This is called the sapwood. It is 

 through this porton of the stem that most of the water, or 

 sap, passes. Girdle this and the food supply of the tree 

 is cut off. 



When the tree is young, all the wood is sapwood. At 

 varying ages in the different species, by some process as 

 yet not thoroughly understood, this sapwood undergoes a 

 change. It ceases to conduct the sap, and is strengthened 

 by certain injections that harden and mature it and 

 darken the color. The change is both chemical and physi- 

 cal. It is then known as heartwood, and practically 

 ceases to play any active part in the life of the tree, save 

 as a mechanical support for the crown. The change 

 from sapwood to heartwood does not take place at any 

 particular age. The sapwood may extend through twenty 

 rings on one side of the tree and only half a dozen on the 

 other ; the line separating the two is very irregular. 



Surrounding the stem is a layer of bark. It varies in 

 thickness from a sixteenth of an inch in the paper birch to 

 six inches in the big trees of California, but is always pres- 



