(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. [Illustration: Fig. 34. Section Thru the Trunk of a Seven Year Old Tree, Showing Relation of Branches to Main Stem. A, B, two branches which were killed after a few years' growth by shading, and which have been overgrown by the annual rings of wood; C, a limb which lived four years, then died and broke off near the stem, leaving the part to the left of XY a "sound" knot, and the part to the right a "dead" knot, which unless rotting sets in, would in time be entirely covered by the growing trunk; D, a branch that has remained alive and has increased in size like the main stem; P, P, pith of both stem and limb.] THE STRUCTURE OF WOOD. REFERENCES:[A] Roth, _Forest Bull._ No. 10, pp. 11-23. Boulger, pp. 1-39. Sickles, pp. 11-20. Pinchot, _Forest Bull._ No. 24, I, pp. 11-24. Keeler, pp. 514-517. Curtis, pp. 62-85. Woodcraft, 15: 3, p. 90. Bitting, _Wood Craft_, 5: 76, 106, 144, 172, (June-Sept. 1906). Ward, pp. 1-38. _Encyc. Brit._, 11th Ed., "Plants," p. 741. Strasburger, pp. 120-144 and Part II, Sec. II. Snow, pp. 7-9, 183. [Footnote A: For general bibliography, see p. 4.] CHAPTER II. PROPERTIES OF WOOD. There are many properties of wood,--some predominant in one species, some in another,--that make it suitable for a great variety of uses. Sometimes it is a combination of properties that gives value to a wood. Among these properties are hygroscopicity, shrinkage, weight, strength, cleavability, elasticity, hardness, and toughness. THE HYGROSCOPICITY[1] OF WOOD. It is evident that water plays a large part in the economy of the tree. It occurs in wood in three different ways: In the sap which fills or partly fills the cavities of the wood cells, in the cell walls which it saturates, and in the live protoplasm, of which it constitutes 90 per cent. The younger the wood, the more water it contains, hence the sap-wood contains much more than the heart-wood, at times even twice as much.