PROPERTIES OF WOOD AND THEIR USES 53 
inches thick, yet from outward 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 sapwood; 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 
