HOW WOOD GROWS

At the very center is the pith. In some trees, this is much softer and possibly a different color than the surrounding heartwood. Heartwood is made up of dead cells that no longer serve any purpose except to support the tree. Next is the sapwood, which carries water, minerals, and plant sugars between the roots and the leaves. This is often lighter in color than the heartwood. Outside the sapwood, close to the surface, is the cambium, a thin layer of living cells. These cells manufacture the wood as they grow. The cambium is covered by a protective layer of bark. The cambium grows rapidly at the beginning of each growing season, creating light colored springwood. As the climate warms, it slows down and produces darker summerwood. This later growth is somewhat denser and harder than the early springwood. As the weather turns cold, the cambium becomes dormant until the next spring. This cycle produces distinctive growth rings.

GRAIN STRUCTURE

As the cambium grows, it generates two types of wood cells. Most of these are long, narrow longitudinal cells that align themselves with the axis of the trunk, limb, or root. These are what give the wood its grain. The cambium also produces a smaller number of ray cells that line up in rays extending out from the pith, perpendicular to the axis.

As the wood grows outward, the living protoplasm inside the cells dies and deteriorates, leaving behind just the cell walls. These walls are composed mostly of cellulose fibers, which give the wood its strength. The fibers are bound together with lignin, a glue-like substance. The hollow longitudinal cells become part of the sapwood, conducting the sap up and down the tree. The hollow rays store plant sugars. After several seasons, the older sapwood turns to heartwood. The sap dries up, and mineral compounds called extractives form on the cell walls. These chemicals turn the wood darker, giving it the characteristic color of its species. They also affect its strength, stability, and hardness.

TYPES OF GRAIN

Because of the manner in which wood grows, every board has a definite grain direction, parallel to the length of the longitudinal cells. The grain appears differently depending on how the board is sawed.

• When you cut a board across the grain (perpendicular to the grain direction and the growth rings), you reveal end grain.
• Cut wood parallel to the grain direction and tangent to the growth rings, and you’ll see plain grain (also called tangential or flat grain).
• Cut it parallel to the grain direction but through the radius of the growth rings to see quarter grain (also referred to as radial grain).
• Both flat grain and quarter grain are sometimes called long grain.

WOOD GRAIN IN LUMBER

Sawyers use several methods to cut up a tree, each of which reveals different grain patterns.

• The most common method is plain sawing because it produces the highest quantity of usable lumber. The sawyer begins by sawing several boards from one side of the log, turns it 90 degrees and saws several more, and continues in this manner “sawing around” the log. Plain-sawn boards show flat grain on their faces and quarter grain on the edges.

• The sawyer might also quarter saw a log. First, he saws the log in quarters, then slices each quarter into boards, either by cutting boards from the two flat sides alternately or by gang-sawing the quarter (making parallel cuts). Quartersawn boards show mostly quarter grain on their faces and flat grain on the edges.

• On special request, a sawyer will live saw a log for a woodworker, gang-sawing the entire log. (This is sometimes called sawing through and through.) Live sawing produces much wider boards than other methods, and these boards show mostly mixed grain — flat grain near the center of the face and quarter grain near the edges.

TEXTURE AND PATTERN

The size, type, and arrangement of the wood cells differ with the species, and this also affects the appearance of the grain. The texture of the wood is determined by the relative size of the longitudinal cells. Wood species with large cells are said to have a coarse texture, while those with smaller cells have a fine texture.

Hardwoods have vessel elements, a special type of longitudinal cell much larger in diameter than the surrounding cells. When these vessels are sliced open, they leave tiny hollows in the wood called pores. These pores give the hardwoods a distinct look from softwoods, which have none. Some hardwoods have a larger concentration of pores in the springwood — these are known as ring-porous woods. Hardwoods in which the pores are distributed evenly throughout the springwood and summerwood are ring-diffuse. The arrangement of pores has an enormous effect on the grain. Ring-porous hardwoods have a pronounced or strong grain pattern, while the grain pattern of ring-diffuse stock is much less distinct.

Hardwood pores also come in a wide range of sizes. Woods with large pores that are easily visible to the naked eye are said to have an open grain. Those with smaller pores, to small to see clearly, have a closed grain. Open-grain woods appear coarser than closed-grain woods because the surface isn't as smooth. When the wood is sawn and the pores are split, the open pores create tiny valleys and rifts. Oftentimes these pores must be filled before the wood is finished so the finish will dry to a smooth, flat surface.

FIGURED WOOD GRAIN

Wood grain isn’t always straight and even. The longitudinal and ray cells sometimes grow in unusual patterns, many of which are strikingly beautiful. These are known as figured grain.

A few wood species, such as white oak, have especially prominent rays. When quartersawn, these produce silver grain.*

Crotch figure, such as this walnut crotch, is cut from the part of a tree where the trunk divides into smaller limbs and branches.*

Curly grain occurs when the longitudinal cells grow in waves. This occurs in many species but is especially striking in maple.*

Bird’s eyes like those in this maple are caused by small dimples in the layers of cells. These are thought to be caused by a fungus that affects the growth of the longitudinal cells.*

Larger dimples result in quilted figure, like the quilting in this soft maple. This, too, is the result of a fungus.*

The longitudinal cells of certain species, such as mahogany, sometimes spiral around the trunk, reversing direction every few growth rings. This creates ribbon figure.*

Sometimes a tree produces a large growth on the side of the trunk or a branch. The cells seem to swirl around each other inside these growths. When sliced, these produce a burl figure such as this elm burl.*