PROPERTIES OF WOOD AND THEIR USES 53 



inches thick, yet from ontward appearances the tree is 

 as healthy as ever because the flow of sap from the roots 

 to the leaves and down again to the different parts of 

 the trunk is not interrupted. However, if a deep notch 

 is cut completely around the outside this is called 

 girdling and the vessels are cut, the flow of sap is 

 interfered with, and the tree promptly dies. 



The thickness and amount of sap and heartwood 

 varies in different species and also in different parts 

 of the same tree. Walnut and red cedar have com- 

 paratively thin sap wood; in the case of hickory, beech 

 and maple it is quite thick, while in other trees, like 

 hemlock, spruce and willow, there is practically no dif- 

 ference in appearance. 



Upon close inspection the cross-section of the stump 

 will show the annual rings which have been formed by 

 the cambium layer. These rings are generally wide in 

 the center of the trunk gradually narrowing as the 

 outside is approached. This relation shows that the tree 

 grew rapidly in youth and later on as the trees in the 

 forest became crowded the diameter growth became 

 smaller owing to reduced light and food. The true 

 grain of wood, however, cannot be appreciated on a 

 cross cut except in the case of certain trees like 

 sycamore and the oaks, where it can be seen that the so- 

 called " medullary rays" are very prominent. These 

 rays are narrow bands of pith which transmit food and 

 water from the living part of the tree to the heart- 

 wood. The full beauty of a finely figured species does 

 not appear until the piece is cut along the radius, thus 

 exposing these narrow bands of pith. Quarter sawed 

 oak is a splendid example of such a figured wood cut 

 on the quarter or along the radius. 



In most trees there is a pronounced difference between 



