Heartwood is dead so far as any change in its cells is concerned. Its purpose is merely to stiffen and support the weight of the tree.
Sapwood, on the other hand, has many active cells which assist in the life processes of the tree, tho only in the outer layer of cells, the cambium, does the actual growing or increasing process take place.
Again examining the end of the log, we shall find bright lines radiating from the center. They are composed of the same substance as the pith or medulla and are called pith or medullary rays. These rays are present in all trees which grow by adding ring upon ring but in some they are hardly visible. The purpose of these horizontal cells is to bind the vertical cells together and to assist in distributing and storing up plant food.
Fig. 197.
[Fig. 197] shows a log cut longitudinally or lengthwise. The lines we call grain, it will be seen, are the edges of the annual rings, the light streaks being an edge view of the spring layer and the dark streaks an edge view of the summer or autumn wood.
Fig. 198.
Knots are formed by the massing or knotting of the fibers of the tree through the growth of a branch. [Fig. 198] shows the manner in which the fibers are turned. This packing of the fibers is what causes a knot to be so much harder than the rest of the wood.