Top, a cross-section through a 75-year-old tree showing the
cambium layer. Middle, growth rings, cambium layer and bark. Bottom, an exploded view of growth rings showing the medullary rays.

The grain of the wood in a tree is made up of a series of longitudinal cells created vertically along the tree's axis. In many species medullary rays, or groups of cells which radiate outward from the core, are formed as well. A tree grows from the roots all the way through the trunk of the tree to the canopy above. However, the main area of the tree which is used for wood for veneer slicing is the bole or trunk of the tree. In this area there are many layers which must be studied to understand the anatomy of the tree.
The bark itself is not used in veneer production, but is composed of an outer layer of dead cells and inner living bark. Beneath the bark layer is the cambium layer. It is this layer which develops cells continually during the growth period and forms the trees rings.
Earlier spring growth is of larger cells with relatively thin walls. Later summer growth is of smaller cells with thicker cell walls. The areas of the larger early growth are generally lighter in color, and much wider than the darker, thin, and small late growth rings. The contrast between these growth rings differentiates them and creates the unique and distinctive appearance of many woods. The grain pattern is different for every species, but since each tree is different, it is sometimes possible for veneer from trees to be easily mistyped. At the center of the area of growth rings is the pith of the tree, or center. Since the tree grows around its outer perimeter, the center of the tree is often dead and darker in color than the outer area. In slicing veneer these dead centers are usually excluded.
As the sapling grows larger, it loses its smaller lower branches. The bases of these branches are covered over by additional rings of growth each year, but small knots can appear in these areas of the tree. These small knots or "pin knots" can form interesting grain features, but they can also be considered very objectionable if larger in size and darker in color. It is not until the tree is sawn or sliced that these early knots are found.
The base or butt of the tree is sometimes sliced for figured grain, as are the inner sections of larger branches which form a crotch. But it is the bole, or straight trunk of the tree, which is the predominant area used for architectural veneers. Although the trunk of a tree may appear straight, most trunks are slightly tapered and often slightly bent. The tree which grows slightly bent has different grain on the compressed side than on the tension side. Certain factors such as strong prevailing winds, sloping sites or the angle of light may be further reasons for a tree to be bent. The appearance of the grain, or "figure," in wood comes from slicing through the growth rings of a tree and depends upon the angle that the wood is sliced.

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