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. 18. 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 1/4" in diameter. A _standard_ knot is sound, but not over 1-1/2" in diameter. A _large_ knot is sound, and over 1-1/2" 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.