Wood Structure & Classification of Trees

The following is excerpted from the third edition of Christian Becksvoort’s “With the Grain: A Craftsman’s Guide to Understanding Wood.” 

It is, above all, succinct, easy to understand and perfectly suited for the furniture-maker. As important as what is in its 160 pages is what is not. It’s not a detailed analysis of cell growth. It is not a heap of tables and equations for figuring truss loads in residential construction. It is decidedly not a scientist’s approach to the material.

Instead, “With the Grain” contains the facts you need to know at the lumberyard, in the woodlot and in the shop. It gives you enough science so you understand how trees grow. It explains the handful of formulas you have to know as a furniture-maker. And it gives you a hearty dose of specific information about North American species that will inspire you. Becksvoort encourages you to use the trees in your neighborhood and makes the case that just because you cannot find catalpa at the lumberyard doesn’t mean it’s not a good furniture wood.

You’ll learn to identify the trees around you from their silhouette, leaves and shoots. And you’ll learn about how these species work in the shop – both their advantages and pitfalls.

Becksvoort then takes you into a detailed discussion of how wood reacts to it environment – the heart of the book. You’ll learn how to calculate and accommodate wood movement with confidence and precision. And you’ll learn how to design furniture assemblies – casework, drawers, doors and moulding – so they will move with the seasons without cracking.

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Wood Structure
Wood, or xylem, is the cellular material that makes up the bulk of the tree. It consists mainly of dead, hollow cells. Chemically, wood is composed of 40-50 percent cellulose, 20-35 percent hemicellulose, and 15-30 percent lignin. Cellulose is a very long, complex molecular chain, which, when broken down, yields the simple sugar glucose. Hemicellulose, closely associated with cellulose, is composed of shorter molecular chains of several types of sugar. Lignin is an intercellular material that bonds the wood fibers together. Aside from these three major components, various extractives are also present in the wood. Although not part of the wood structure, the extractives are usually found mostly in the cell walls, and contribute to the wood’s characteristic color, odor, taste, decay resistance and flammability. These secondary ingredients include oils, tannins, waxes, gums, starches, alkaloids, color materials and about 1-2 percent ash-forming minerals such as calcium and silica.

Figure 1-1 shows a typical cross section of a hardwood stem. (A) is the cambium layer, only a few cell layers thick, which produces the bark (phloem) toward the outside of the tree, and wood (xylem) toward the inside of the tree. Cambium extends in a continuous layer between the wood and bark, and gives rise to the tree’s lateral growth. In the temperate zones, during the course of one growing season, one new ring of wood is added to the trunk, roots and branches. Wood formed the previous year does not continue to grow. The width of the growth rings is not necessarily consistent in size, even within a species. Ash, for example, when grown in an area of little rainfall and poor soil, may have up to 40 rings per inch (2.5 cm), slow growth, while the same species grown in an area of abundant moisture and nutrients may have only 4 or 5 rings per inch, or fast growth. Slow versus fast growth also depends on competition from neighboring trees. (B) is the newly formed inner bark, a soft, fibrous material that carries food down from the leaves to all parts of the tree. During the growing season, the cambium layer and the inner bark are very soft and fragile. This means that logs cut during the spring and summer will have easily peeled bark, whereas those cut during the late fall and winter will usually retain their bark. (C) is the dead, corky, outer bark. As the diameter of the tree increases year by year, the bark becomes thicker and begins to crack. With age it usually becomes quite thick and furrowed (except in trees like beech and birch), and begins to slough off from the action of wind and weather.

The newly formed wood is the sapwood (D). It constitutes from one or two to more than 200 years growth (in some conifers), depending on the species (Fig. 1-2). Sapwood functions primarily to conduct water and minerals from the roots to the leaves. Most of the newly formed cells die in a short time. A few, the parenchyma cells, retain their protoplasm in the cell cavity and act as living food-storage cells. After a number of years, the sapwood loses its function and turns to heartwood (Fig. 1-1 E). The last of the parenchyma cells die, leaving only the cell walls that give the tree structural support. Extractives are deposited in the heartwood cells, giving the wood its distinctive color and other properties. Sapwood, having no extractives (some of which are toxic to fungi), is not as decay-resistant as heartwood, and should not be used under conditions conducive to decay. The extractives present in the heartwood of some species add to the mass of the cells, and increase the density of the wood somewhat.

The pith is located in the center of the tree (Fig. 1-1 F). This is a soft, sometimes spongy material formed behind the apical meristem. In most species it is only slightly visible as a darker tube. In some trees such as black walnut (Fig. 1-3), butternut and sumac, the pith is quite pronounced. Surrounding the pith is an indistinct area known as juvenile, or pith wood, characterized by wide growth rings (especially in conifers), low density and strength, and greater longitudinal shrinkage.

Radiating from the pith outward are the rays (Fig. 1-1 G). They extend to the cambium and are used in lateral transport of nutrients through the sapwood. Rays tend to bind the wood, thereby reducing the radial dimensional change.

Trees grown in temperate climates have a cycle of growth and dormancy that results in the appearance of annual rings. Typically, each spring as growth resumes, the cambial layer produces an abundance of large, thin-walled cells. Spring is a time of mild temperatures, maximum daylight and ample rainfall. Consequently, cell growth is quick and profuse. This initial growth is known as early-wood, or spring-wood. As the season progresses and day length decreases, the weather gets hotter and drier and growth slows. Cell formations become smaller and thicker-walled. These are the late-wood cells. Finally, during autumn growth stops for the season. Because the late-wood cells have thicker walls that portion of the growth ring is more dense. This is most apparent in old, weathered wood, which takes on a ridged appearance as the early-wood erodes faster than the dense late-wood.

Some tropical woods have no apparent growth rings. In areas where the weather is constant yearround, growth continues uninterrupted.

Classification of Trees
Botanically, trees are divided into two classes: gymnosperms and angiosperms. Commercially, woods are divided into softwoods and hardwoods, softwoods referring to the gymnosperms and hardwoods referring to the angiosperms. These terms should not be taken literally, because not all softwoods are soft (yellow pine is about as hard as black walnut), nor are all hardwoods hard (basswood, and cottonwood are quite soft). Because these terms will continue to be used in the trade, the knowledgeable woodworker should recognize that they are used merely to distinguish the two groups, not define them.

Gymnosperms, characterized by exposed seeds, are the older of the two groups, and include all the conifers and even gingko (commonly thought of as a hardwood because of its broad, deciduous leaves). Conifers are characterized by their single, straight trunk and needle-shaped leaves, which are retained all year (except for cypresses and larches). Needles, usually smaller than leaves, have a waxlike coating that prevents water loss during long periods of dormancy. Thus adapted, the conifers are the dominant trees in the northern forest.

Coniferous wood is fairly simple (primitive) in structure. It is composed predominantly (roughly 90 percent) of all-purpose tracheid cells, which are long, thin and hollow. Although the cells are closed at both ends, liquids pass from one cell to the next through “pits” in the cell walls. Conifers also contain medullary rays. Some of the conifers also contain resin canals, which are large (visible), tubular passages that exude resin, or pitch. These canals occur in the species of pine (Pinus), larch (Larix), spruce (Picea) and Douglas fir (Pseudotsuga). Conifers lack specialized vessels or pores. The entire group is classified as having nonporous wood.

On a cellular level, the angiosperms are more specialized than the gymnosperms. They contain more ray cells (from 6 percent to 31 percent by volume), and only about 25 percent of the wood consists of fibers. These, like the tracheids in conifers, are long, thin cells with thick walls that give the tree support. From 6 percent to 55 percent of the wood is composed of vessels, or tubular openended cells. They have thin walls and large diameters, specifically made for the conduction of sap. When the vessels are exposed by cutting, they are called pores. Wood with large vessels such as oak, ash or chestnut are said to be open-pored, or opengrained. When exposed on any surface, these vessels are readily visible by eye or with a hand lens, and can be felt by a fingernail across what appears to be a smooth surface. Because vessel sizes vary widely among species, designation such as open- or close-grain are relative. What is more important is the arrangement of the vessels in the growth rings.

When large vessels (pores) are formed primarily in the early-wood, the wood is called ring-porous. The vessels form a distinct band in the early-wood (Fig. 1-4). Vessels, although present in the late-wood, are much smaller and fewer. Ring-porous woods include ash, hickory, oaks, chestnut, elm, sassafras and locust.

At the other extreme are the woods that are diffuse porous (Fig.1-5), those with pores scattered evenly throughout the year’s growth. The pores are all about equal in size, so the early-wood and late-wood are therefore indistinct. Maple, birch, dogwood, beech, holly, poplar, magnolia, hornbeam, sycamore, willow and basswood are all diffuse-porous.

Between these two distinct groups are the semi-ring porous (or semi-diffuse porous) woods (Fig. 1-6). The main characteristic of these woods is a gradual change from large vessels in the earlywood to smaller vessels in the late-wood. Species that have semi-ring porous wood include catalpa, persimmon and plum. Certain species of hickory, walnut, oak and willow are sometimes included in this group.

The pores of exposed end-grain heartwood will sometimes appear clogged with a frothy, film-like substance called tyloses. Tyloses is formed in some species when the sapwood turns to heartwood. In woods such as red oak, cherry, maple, dogwood and honeylocust tlyloses is insignificant or totally absent. In others such as white oak, tyloses development is quite extensive, while in Osage orange and black locust, the vessels are tightly filled.