Conceptual Overview

Wood, in the conventional sense, comprises about 90% of a tree trunk, and the remaining 10% is occupied by the bark which includes components of phloem, cortex and periderm; however, in the anatomical sense, the term "wood" is usually used as a synonym for the secondary xylem of woody plants. In the plant, the wood performs water-conducting, supportive, and storage functions. A tree is basically held up by its wood. However, in different plants wood may differ greatly in structural properties, and therefore requires special processing to be used for different purposes.

The anatomical features of different woods underlie their structural properties and their utilization. The anatomical characteristics of woods are very stable and distinctive for different taxa. The investigation of structure of even a very small piece of wood is often quite sufficient in order to identify the plant. This is why xylotomy (dealing with the comparative study of different woods) exists as a specialization of plant anatomy. Although the anatomical structure of wood in different plant taxa varies greatly, wood in general still has many structural characters in common.

One of the most distinctive anatomical features of all woods is that their cells are arranged in a very ordered series due to their principal origin from tangentially dividing cambial cells. All secondary walls of woods are lignified and have a variety of pits. In all plants, wood is comprised of two systems: axial (vertical) and radial (horizontal). Elements of the axial system (tracheids, vessels, fibers and axial wood parenchyma) are derivatives of fusiform cambial cells and axially elongated. Although axial parenchyma cells are more or less isodiametric, they are formed from the same fusiform cambial cells as vessels, tracheids and fibers, but as a result of subsequent transverse divisions. Therefore, their derivatives form vertical cell tiers. A radial system is made up of ray cells that are the derivatives of ray initials in the vascular cambium. Naturally, these two systems appear differently depending on the plane of sectioning.

Woods are studied in three planes of section: transverse (cut at a right angle to the main axis), longitudinal radial (vertical but along the radial axis and passing through or near the center of the stem), and longitudinal tangential (perpendicular to a radius), as well as in macerated preparations. Fibers and tracheary elements are usually dead, and therefore empty at maturity, whereas both axial and ray parenchyma are made up of living cells.

Characteristic of woods are growth rings (also called growth layers or annual rings), with the earliest ring being in the center of the stem, and the most recent ring in the wood adjacent to the vascular cambium. The rings are evident due to periodical (commonly seasonal) activity of the cambium, and to the more or less sharp boundary between wood produced during slowing down of cambial activity (late wood) and that wood deposited after cambial reactivation (early wood). Of course, in any growth ring, late wood lies outside early wood. The age of a woody stem can be determined by counting growth rings. The cells of the early wood (also called spring wood) are generally larger in diameter, and thinner-walled than those of late wood (summer wood). The width of the growth ring, and the relative amount of early and late wood, are largely dependent on the environmental growth conditions. Adverse growth conditions tend to decrease ring width, primarily due to the preferential decrease of early wood deposition.

Examination of the cut end of a log often shows the presence of two zones: an outer,usually light-colored zone, called the sapwood, and an inner zone, which is usually darker in color, called the heartwood. The sapwood functions in water conduction and in food storage. In conifer tracheids living parenchyma cells die off during the transformation of sapwood into heartwood, and water movement along tracheary elements is subsequently blocked by the aspiration of bordered pit membranes.

The heartwood is dead and non-conductive. It becomes darker in color with the accumulation of tannins, resins, gums, oils, polyphenolics and other complex compounds representing by-products of metabolism. Some of these substances are believed to be toxic to fungi, bacteria, and some insects, or to repel them.

In common usage, woods of conifers are referred to as softwoods (no xylary fibers), and that of angiosperms is termed hardwood (due to the presence of xylary fibers). This classification has little relation to the actual hardness of the woods, e.g. southern hard pine (Pinus palustris) is quite hard, while eastern white pine (Pinus strobus) is quite soft by comparison. Both are, of course, conifers.

Knots in wood are the xylem of branches. Dead branches in lumber usually form loose knots and may be faulty due to the growth of fungi in dead limbs and bark. Living branches that appear in lumber are generally called hard knots, and indeed stay in place more readily than loose knots.

In some dicotyledons, plugging of vessel elements occurs by means of tyloses (pl.). A tylosis (sing.) is an outgrowth of a living ray cell or axial parenchyma cell into the cavity of a vessel through a half-bordered pit membrane. The tyloses are thin-walled and unlignified although they sometimes may develop thick, secondary walls. Compounds of various sorts, including tannins, gums, resins, organic acid salts and other substances, are often secreted by living parenchyma cells at the terminal stage of their life-cycle. These depositions may profoundly change wood properties. Thus, while development of old growth rings progressively is switched off, younger rings produced by vascular cambium are switched on.

A specialized wood, termed reaction wood, develops in large horizontal branches and leaning stems of trees as a result of the uneven pressure exerted on the upper and lower sides. In dicotyledons, reaction wood develops in the upper sides of branches (called tension wood). Its vessels are reduced in number and diameter and the walls of libriform fibers have a thick cellulosic inner layer, a so-called gelatinous fibers.

The woods of conifers are fairly simple in their anatomy. The major mass of wood consists of tracheids. Libriform fibers, axial parenchyma and vessels are absent. Thus, this wood is referred to as "non-porous" meaning that there are no vessels. Axial parenchyma is abundant in some conifers and poorly developed in others. In some cases, typical of certain taxa, only the cells of vertical resin ducts may be represented. Rays, as seen in transverse and tangential sections, are mostly one-cell wide (i.e. uniseriate). Only in horizontal resin ducts are they found to be multiseriate. In tangential section, rays appear as strips from one to twenty cells high.

The woods of dicotyledons are much more complex due primarily to the presence of vessels, axial parenchyma and libriform fibers, although some primitive taxa have non-porous woods. The structure, size, shape,type and arrangement of wood elements are taxon specific and can be used as diagnostic features. Two major groups are recognized according to the size and distribution of vessels across ring growth intransverse section of woods: ring-porous and diffuse-porous. In ring-porous wood, vessels are of different diameters and the largest ones are concentrated in the early wood creating the appearance of a "ring." In diffuse-porous wood, vessels are more or less uniform in diameter and uniformly distributed throughout the growth ring.

The structure of perforation plates (e.g. simple or scalariform with either few or numerous bars), and that of the lateral walls (the pattern of pittings and wall thickenings on the inside of the secondary wall) in vessels, and the pattern of libriform fiber pittings are also used for wood identification.

Other important diagnostic features (primarily for angiosperms) are the amount and distribution of the axial parenchyma. In some species it is entirely absent or occurs in very small amount, while in others axial parenchyma is very abundant. It may be distributed evenly or aggregated in large tangential bands. Of particular diagnostic importance is the spatial relationship of axial parenchyma to vessels. There exists apotracheal parenchyma, which is not associated specifically with the vessels, and paratracheal parenchyma, which surrounds vessels in different patterns. The structure and composition of rays are also used for wood identification. Still another useful technique often applied in wood anatomy is maceration, which may allow for a more detailed study of cell types in wood by digesting away the middle lamella and thus permitting disintegration of wood into individual cells that can be viewed intact.

Subunits:		Features:
Conifer Woods (Softwoods)		Wood Key
Dicotyledon Woods (Hardwoods)		Video: Pulp & Paper