During the development of a tree most of the limbs, especially the lower ones, die, but persist for a time--often for years. Subsequent layers of growth of the stem are no longer intimately joined with the dead limb, but are laid around it. Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber. In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place.[32] [Footnote 32: See Standard classification of structural timber. Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303. Contains three plates showing standard defects.] Knots materially affect checking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration. The weakening effect is much more serious where timber is subjected to bending and tension than where under compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress. Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects. Extensive experiments by the U.S. Forest Service[33] indicate the following effects of knots on structural timbers: [Footnote 33: Bul. 108, pp. 52 _et seq._] (1) Knots do not materially influence the stiffness of structural timber. (2) Only defects of the most serious character affect the elastic limit of beams. Stiffness and elastic strength are more dependent upon the quality of the wood fibre than upon defects in the beam. (3) The effect of knots is to reduce the difference between the fibre stress at elastic limit and the modulus of rupture of beams. The breaking strength is very susceptible to defects. (4) Sound knots do not weaken wood when subject to compression parallel to the grain.[34] [Footnote 34: Bul. 115, U.S. Forest Service: Mechanical properties of western hemlock, p. 20.] FROST SPLITS