Fig. 32.
Figure Formed by Pith Rays in Oak (full size).
Fig. 33.
Sweet Gum, Showing Uneven Deposit of Coloring Matter (full size.)
The presence of a limb constitutes a knot and makes great irregularity in the grain of wood, Fig. 34. In the first place, the fibers on the upper and lower sides of the limb behave differently, those on the lower side running uninterruptedly from the stem into the limb, while on the upper side the fibers bend aside making an imperfect connection. Consequently to split a knot it is always necessary to start the split from the lower side. On the other hand it is easier to split around a knot than thru it. The texture as well as the grain of wood is modified by the presence of a branch. The wood in and around a knot is much harder than the main body of the trunk on account of the crowding together of the elements. Knots are the remnants of branches left in the trunk. These once had all the parts of the trunk itself, namely bark, cambium, wood, and pith. Normally, branches grow from the pith, tho some trees, as Jack pine and redwood, among the conifers, and most of the broad-leaf trees have the power of putting out at any time adventitious buds which may develop into branches. When a branch dies, the annual layer of wood no longer grows upon it, but the successive layers of wood on the trunk itself close tighter and tighter around it, until it is broken off. Then, unless it has begun to decay, it is successively overgrown by annual layers, so that no sign of it appears until the trunk is cut open. A large trunk perfectly clean of branches on the outside may have many knots around its center, remnants of branches which grew there in its youth, as in Fig. 34, and Fig. 8, [p. 19]. The general effect of the presence of a knot is, that the fibers that grow around and over it are bent, and this, of course, produces crooked grain.
Following are the designations given to different knots by lumbermen: A sound knot is one which is solid across its face and is as hard as the wood surrounding it and fixed in position. A pin knot is sound, but not over ¼" in diameter. A standard knot is sound, but not over 1½" in diameter. A large knot is sound, and over 1½" in diameter. A spike knot is one sawn in a lengthwise position. A dead, or, loose knot is one not firmly held in place by growth or position.
(4) Pith. At the center or axis of the tree is the pith or medulla, Fig. 34. In every bud, that is, at the apex of every stem and branch, the pith is the growing part; but as the stem lengthens and becomes overgrown by successive layers of wood the pith loses its vital function. It does not grow with the plant except at the buds. It varies in thickness, being very small,—hardly more than 1/16", in cedar and larch,—and so small in oak as to be hardly discernible; and what there is of it turns hard and dark. In herbs and shoots it is relatively large, Fig. 5, [p. 15], in a three-year old shoot of elder, for example, being as wide as the wood. In elder, moreover, it dies early and pulverizes, leaving the stem hollow. Its function is one of only temporary value to the plant.
Fig. 34.