It is formed by thickening growth,which persistently adds newlayers that accumulate as a cylinder of wood between the pith and the bark.
Technically, wood is secondary xylem. The growth in girth that produces it is called secondary growth. This growth occurs after the stem or root segment has completed its increase in length, or primary growth. Secondary growth also yields, in much smaller amounts, secondary phloem, which becomes a part of the bark.
Secondary growth results from divisions of the cells in a layer called the vascular cambium, located between the bark and wood. Cambial cell divisions that are oriented toward the interior of the stem or root (that is, on the pith side of the cambium) result in a new layer of secondary xylem cells at the periphery of the layers formed previously.
Cambial cell divisions oriented toward the exterior (on the bark side) of the stemor root add a new layer of secondary phloem to the inside of the bark. The angiospermous group Monocotelydones (the monocots) forms neither secondary xylem nor secondary phloem, because the fibrous conducting tissues are arranged in scattered bundles rather than in layers.
Wood Structure and Growth Rings
Wood is composed of cells arranged in two orientational systems. Most of the cells are oriented axially, in approximate alignment with the long axis of the stem or root. The rest are oriented radially, perpendicular to the long axis. The axial, or longitudinal, cellular system functions in both support and water conduction.
In conifers, which include pines, spruces, and firs, this system is composed mainly of tracheids, with some parenchyma cells. In angiosperms, which include oaks, ashes, and elms, the axial system contains, in addition to tracheids and parenchyma, vessel members and fibers.
Vessel members are one of angiosperm wood’s most distinctive features because they commonly enlarge greatly in diameter. The radial system in both gymnosperms and angiosperms is composed mainly of parenchyma cells. These are aggregated into rays,which conduct nutrients from the phloem to the interior of the stem or root.
In temperate-zone trees, the vascular cambium usually lays down a single increment of secondary xylem each year, during the warm season. The wood formed early in the growing season, called early wood, is less dense, and in some species has a different cell composition than the latewood.
Because of the within-increment contrast in cell characteristics, the increments appear as annual growth rings on cross sections. Trees growing in relatively uniformtropical climates do not form annual rings because growth is not generally limited to a particular season.
Softwoods and Hardwoods
Angiosperm woods are often referred to as hardwoods and coniferous woods as softwoods.Although these terms are generally accurate, some hardwoods are actually softer than some softwoods.
For example, the conifers known as “hard pines” are technically “softwoods,” and basswood is technically a “hardwood,” yet hard pines are much harder than basswood. Softwoods compose much of the commercial lumber in the Northern Hemisphere.
Sapwood and Heartwood
Because each new increment of wood is produced at the periphery of the preceding increment, the wood that is nearest the bark is the youngest. Eventually, the older wood,which is deeper within the tree, loses its capacity to function in conduction and storage. It accumulates oils, resins, tannins, and other substances.
This darker, nonconducting, inner wood is called heartwood. The lighter, functioning, outer wood that surrounds it is called sapwood. The relative amounts of heartwood and sapwood, and the degree of their color contrast, vary with the species.
Leaning branches or trunks produce a specialized kind of wood, called reaction wood, that generally helps the stem return to a more vertical orientation. In conifers, reaction wood develops on the undersides of leaning branches or trunks and is called compression wood.
In angiosperms, it develops on the upper sides and is called tension wood. In leaning conifer stems, the growth rings are much wider on the compression-wood side of the stem than in the ordinary wood on the opposite side.
Certain characteristics that affect wood’s appearance are quite variable. Color is one. For example, black walnut, widely considered North America’s finest cabinet wood, owes its aesthetic appeal largely to its heartwood, which is often a rich chocolate brown, in contrast to the much lighter heartwood of many other species. Grain, texture, and figure also affect the appearance of wood.
Grain is considered straight if the longitudinal wood cells closely parallel the stem’s long axis. Deviations produce spiral, wavy, and interlocked grains. Texture,which refers to the sizes and proportional distribution of the various kinds of wood cells,may be coarse or fine.
Figure, or distinctive markings and patterns on longitudinal wood surfaces, results from basic anatomical structure, irregular coloration, and defects or from irregular patterns as in “bird’s-eye maple.”
During the course of secondary growth, the bases of the branches that had formed when a tree was younger are typically buried within the trunk or within a larger branch, through addition of successive increments of woody tissue. That part of a branch that becomes overgrown by secondary growth is called a knot. Knots generally degrade the quality and value of lumber.