Wood Structure and Chemical Composition

Abstract

Wood is extremely nonhomogeneous, and its structural and chemical variability is reflected in wide ranges in its physical properties such as permeability, capillary behavior, thermal conductivity, and the diffusion of bound water. The greater uniformity is found among the woods of conifers or softwoods. Those of dicotyledonous angiosperms or hardwoods, on the other hand, have extreme structural differences and are therefore much easier to distinguish from one another visually. Hardwood species are also much more numerous because they occur in tropical as well as temperate regions, while softwoods are found primarily in the temperate zones. Because of greater structural variation the hardwoods have a greater range in permeability and capillary behavior. In both types earlywood is generally of much lower density than latewood. The earlywood of dried softwoods is usually less permeable than the latewood while, in hardwoods, the reverse is commonly true owing to the larger earlywood vessels. There can also be significant differences between stands of trees of the same species, within a tree, between boards, and within a board. Heartwood is nearly always much less permeable than sapwood due to pit aspiration, incrustation, and vessel tyloses in the former.