Wood Cells

The cells which make up the structural elements of wood are generally tubular and quite firmly grown together. Dry wood cells may be empty, or partly filled with deposits such as gums and resins.

Many wood cells are considerably elongated and pointed at the ends. Such cells are called fibers. The direction of the wood fibers with respect to the axis of the tree is one of the most important characteristics affecting the usefulness of a given piece of wood, since it has a marked influence on strength.

The length of wood fibers may vary considerably in a given tree, as well as between different species. Typically, hardwood fibers average about l/25 inch in length; softwood fibers average from l/8 to l/4 inch in length, or longer.

Growth Rings

Between the bark of a tree and the wood interior is a layer of thin-walled, nearly invisible living cells, called the cambium layer, in which all growth of the tree takes place. New wood cells are formed on the inside and new bark on the outside of the cambium. No growth in either thickness or length takes place in wood already formed, new growth is purely the addition of new cells -- not the further development of old ones.

In temperate climates there is usually enough difference in color and texture between the wood formed early and that formed late in the growing season to produce well-marked annual growth rings. The age of a tree at any cross-section may be determined by counting the growth rings. One ring represents one year of growth, provided the growth has been interrupted by cold or dry seasons so that the change in cell structure is sufficient to define the annual layer.

Springwood and Summerwood

In many species of wood each annual growth ring is divided into two distinct layers. The inner part of the ring, formed first in the growing season, is called springwood. The outer part, formed later in the growing season, is called summerwood.  The transition from springwood may be gradual or abrupt, depending on the kind of wood and the growing conditions when it was formed.

Springwood is generally characterized by cells with relatively large cavities and thin walls, whereas summerwood cells have smaller cavities and thicker walls. Summerwood generally will be heavier, harder and stronger than springwood.

The percentage of summerwood in a given piece of lumber determines the density of the piece. Other factors being equal, the higher the density, the greater the strength.  Because of its greater density, the proportion of summerwood in a particular piece of lumber is sometimes used as an indication of its quality and strength.

Sapwood and Heartwood

The wood portion of a tree has two main parts. The outer part, which consists of a ring of wood around the tree just under the bark, is called sapwood. Within the sapwood ring is an inner core, generally darker in color, called heartwood.

The sapwood ring varies in thickness depending on the age and specie of the tree. Sapwood contains the living cells and takes part in the active life processes of the tree. Heartwood consists of inactive (not dead) cells and serves mainly to give strength to the tree. Except for the slightly or sometimes distinctly darker color of heartwood, there is often little difference in the strength or physical characteristics of heartwood and sapwood from a given tree.

As a tree grows older and larger, the inner layers of sapwood change to heartwood. Eventually the heartwood core forms the major part of the trunk and main branches.

Chemical Composition of Wood

Wood is a complex aggregate of compounds which may be divided into two major groups: (1) those which make up the cell structure, and (2) all other substances, which are commonly called "extractives" or infiltrated materials.

The cell wall components consist primarily of cellulose and lignin. Cellulose is the most abundant constituent, comprising about 70 to 80 percent of the wood structure. Lignin, which comprises from 20 to 30 percent of the wood structure, is the cementing agent which binds the individual wood fibers together to form a substance of strength and rigidity.  In addition to cellulose and lignin, wood contains a small amount of mineral matter. These minerals, known as "ash forming" minerals because they are left as ash when the lignin and cellulose are burned, constitute less than one percent of the total wood substance.

The extractives are not part of the wood structure as such, but they contribute such properties as color, odor, taste and resistance to decay. They include tannins, starches, oils, resins, acids, fats and waxes. They are found within the hollow portions of the wood cells, and can be removed from the wood by neutral solvents such as water, alcohol, benzol, acetone and ether.

Hardwoods and Softwoods

All wood species are classified for commercial purposes as either hardwoods or softwoods.

The hardwoods are the broad-leafed, deciduous trees which drop their leaves at the end of the growing season. Examples of commercially-grown hardwoodtrees include oak, maple, walnut and ash.

The softwoods are evergreen trees. Evergreen trees may have fern-like leaves, typical of the redwoods, or needle-shaped leaves typical of the pines and firs. Softwoods are also known as conifers (or "cone bearers") because all softwood trees bear cones of one kind or another.

The terms "hardwood" and "softwood" are somewhat misleading in that they have no direct application to the actual or relative hardness or softness of a particular kind of wood.Many hardwoods are softer than the average softwood. Douglas Fir, which is widely used in the west as a construction material, is a softwood by definition; nevertheless, the better grades of Douglas Fir are dense, hard and tough.

Specific Gravity

Although dry wood of most species will float in water, the absolute specific gravity of the basic substance of which wood is composed is about 1.55 for all species. Thus it is evident that a large part of the volume of wood is occupied by cell cavities and pores, so that the resultant relative specific gravity of wood is less than 1.00 for most species.

Variation in the size of the cell openings and the thickness of the cell walls causes some species to have more wood substance than others and therefore to have higher relative specific gravities. Consequently, the density of cut lumber will vary between species, averaging from 30 to 40 pounds per cubic foot at normal moisture content for most commercially grown softwoods.

Since density depends on the amount of wood substance in a given piece of lumber, it is an excellent index of strength.  The higher the density, the greater the strength of cut lumber, all other factors being equal.

Grain

The term "grain" as it is applied to wood is most often used to indicate the direction of the wood fibers relative to the axis of the tree or the longitudinal edges of a piece of cut lumber. Thus, if the fibers are generally parallel to the axis of a tree, the lumber from the tree will be straight-grained; however, if the direction of the fibers makes an angle with the axis, the lumber will be cross-grained.  (Note that the term "cross-grain" is also used to indicate a direction which is actually perpendicular to the grain.  This usage is generally associated with the direction, with respect to the grain, at which a load is applied.)

Grain is also used in reference to the width and spacing of the annual growth rings. Thus, lumber may be close-grained, medium-grained or coarse-grained. Note, however, that these are relative terms without precise meaning.

Edge grain refers to lumber in which the growth rings are at approximately right angles to the surface of the piece. Flat grain refers to lumber in which the surface of the piece of lumber is approximately tangent or parallel to the direction of the growth rings.

Moisture Content

Living trees may contain as much as 200 percent moisture by weight. After a tree is cut and converted into lumber, the wood begins to lose moisture. The process of removing moisture from green lumber is known as seasoning, which may be accomplished by exposure to the air or by kiln drying.

Green wood contains moisture in two forms: as "free water" in the cell cavities and as "absorbed water" in the capillaries of the cell walls. When green wood begins to lose water, the cell walls remain saturated until the free water has evaporated. The point at which evaporation of free water is complete and the cell walls begin to lose their moisture is called the "fiber saturation point." The fiber saturation point occurs at a moisture content of about 25 to 30 percent for most species.

Variations in moisture content above the fiber saturation point have no effect on the volume or strength of wood. as wood dries below the fiber saturation point and begins to lose moisture from the cell walls, shrinkage begins and strength increases.

Wood in use over a period of time will give off or take on moisture from the surrounding atmosphere until the moisture in the wood corresponds to the humidity of the surrounding atmosphere. When exposed to similar atmospheric conditions, different woods will have the same moisture content regardless of their density.

Moisture content has an important effect on susceptibility to decay. Most decay-producing fungi require a moisture content above the fiber saturation point to survive. In addition, favorable temperatures, an adequate supply of air and a source of food are essential. Wood that is continuously water-soaked (as when submerged) or is continuously dry (i.e., with a moisture content of 20 percent or less) will not decay.

Shrinkage

Shrinkage of wood takes place between the fiber saturation point and the oven-dry condition. It is stated as a percentage of the original or green dimension.  Wood shrinkage is greatest in the direction of the annual growth rings, somewhat less across the rings, and very little along the grain. Typically, shrinkage along the grain (longitudinal shrinkage) is usually less than one percent and therefore too small to be of practical significance.


Shrinkage of commercial softwood boards across the grain averages about one percent for each four-percent change in moisture content. Shrinkage of hardwoods is slightly larger.  Large structural timbers shrink proportionally less than smaller pieces of lumber because drying does not take place simultaneously in the inner and outer portions. The inner portion dries more slowly than the outer portion and prevents the wood near the surface from shrinking normally. Later, when drying of the interior occurs, the outer portion which has now set prevents the inner portion from shrinking to the extent that it otherwise would.

End.