Differential growth rate: In the photo above, the bottom piece of oak grew one inch in four years; the top piece took 10 years to grow the same amount. One result is a very different grain pattern on the surface. The bottom piece will display lines wide apart; the upper piece, close together.

End grain structure magnified 10 times

Red Oak

White Oak

African Mahogany

A tree has three basic parts. The roots secure the tree in the ground and take in groundwater containing mineral salts from the soil. The trunk transports this solution, called sap, from the roots to the leaves; it stores food; it holds the living cell layers essential to the growth of the tree; and it provides rigidity to the crown — the smaller branches and twigs on which the leaves grow. The leaves absorb carbon dioxide from the air, give off oxygen and, by photosynthesis, enrich the sap with sugars, which are passed down the inner bark and used to promote growth.

How Wood Forms

Just below the bark is a microscopically thin layer of living cells called the cambium that sheathes the tree from ground to crown. The cambium cells grow and divide. Half of the cells make either wood or bark; the other half remain in the cambium to grow and divide again. New cells on the inside of the cambium become one of the woody elements. Cells on the outside become bark, which is divided into two layers. The inner bark carries the sugar-rich sap down from the leaves to feed the cambium and roots. The outer bark protects the fragile cambium from invasion by insects, fungi, animals and extremes of heat and cold.

A tree grows by putting on a layer of wood underneath the bark each year. In temperate climates, most growth occurs in the spring and summer, when the leaves are busy converting sunlight into food.

When you look, for example, at a board of quartersawn oak (see Chapter 8, Lesson 2, for an explanation of what quartersawn means), you see three types of wood tissue. The distinct lines come from tissue laid down in the early part of the growing season; they’re called vessels. The plain, dense material between these lines is laid down later in the growing season; it’s called fiber. The stuff smudged across these two lines are rays.

Vessels are essentially food transportation tubes with thin walls surrounding a wide center hole. (In temperate regions where there is a distinct growing and resting period, vessels of many tree species form a clear growth ring.) Fiber consists of tubes with a very small center hole surrounded by very thick walls that strengthen and support the tree.

Ray tissue is present in all species, although it is more pronounced in some. It’s hardly visible in maple, for example, but clearly visible in oak. On the end grain of a log or wide board, the
rays can be seen radiating out from the center like the spokes in a wheel. The rays are storage tissue shaped somewhat like tea plates embedded in the wood. On the end grain of a board or a log you are seeing the plates cut in two. In oak they are particularly thick plates, so when the board is quartersawn, exposing the flat of the plate on the surface, the rays appear as large, bright areas going across the grain. They are referred to as “ray fleck,” “flake” or "figure.”

For a host of reasons, including soil attributes, weather and the proximity of other trees, not all trees grow at the same rate; this is called differential growth rate. In addition, some trees grow in a spiral form. (You can see this in the surface cracks on a wooden utility pole.) This growth pattern increases the tree’s resistance to stress, but it also poses problems for the woodworker when machining the converted board.

After five to 10 years of growth, the wood in the center of all trees undergoes a chemically complex change, transforming into the non-living heartwood of the tree. It usually is darker and harder than the younger sapwood, which is the living outer portion of wood between the cambium and the heartwood. In most trees, the transition from sapwood to heartwood is obvious because of the color change. In some, the pale color of the heartwood is hardly distinguishable from the pale color of the softwood.

Grain, Texture and Figure

Some confusion surrounds what is meant by grain, texture and figure when used to describe the wood surface. Grain refers to the wood fibers relative to the length of the tree on the faces and edges of a piece of timber. Texture is the relative size and variations of the elements. Figure refers to the pattern on a board caused by the arrangement of the different elements and the nature of the grain.

Softwood and Hardwood

Trees are popularly categorized as softwood or hardwood. These designations have to do with the leaf type of the tree and don’t necessarily reflect the actual hardness or softness of the wood. Softwood comes from needle-bearing trees; hardwood comes from broad-leaved trees.