Wood is a naturally occurring organic material formed by the secondary growth of trees and other woody plants. It serves the primary functions of mechanical support and water transport from the roots to the leaves. Understanding wood’s inner architecture, from its chemical building blocks to its visible grain, allows for a better prediction of how it will behave in different applications. The structure determines everything from the strength of a beam to how lumber responds to changes in atmospheric humidity.
The Chemical and Cellular Makeup
Wood’s strength and rigidity originate at the microscopic level with the composition of its cell walls, which are built from three main polymers. Cellulose is the most abundant organic substance, making up 40 to 50 percent of its dry weight. It forms long, chain-like molecules that aggregate into strong microfibrils, providing the wood with its tensile strength.
The spaces between these cellulose microfibrils are filled by hemicellulose and lignin. Hemicellulose is a partly crystalline polymer that acts as a binder. Lignin is a complex, amorphous polymer that provides rigidity, cementing the cellulose fibers together. Lignin accounts for 26 to 32 percent of the dry mass in softwoods and 20 to 28 percent in hardwoods. This combination creates a rigid, supportive structure.
The structural components are organized into microscopic cells. The most common are elongated, tube-like elements that align vertically along the trunk. These axial cells, often called fibers or tracheids, provide mechanical support and conduct water. Wood also contains brick-like parenchyma cells grouped into rays that run horizontally from the center outward. Ray cells primarily store and radially transport food reserves.
Macro Structure and Growth Patterns
The visible structure of wood results from the tree’s annual growth cycle, producing distinct growth rings. Each ring contains two zones: earlywood and latewood, reflecting changing seasonal conditions. Earlywood forms during rapid spring growth, resulting in cells with larger cavities and thinner walls, making the wood lighter and less dense.
As growth slows in late summer and autumn, the tree produces latewood. These cells have smaller cavities and thicker walls, resulting in a denser, stronger structure. The sharp boundary between the dense latewood and the less dense earlywood of the next year creates the visible ring pattern used to estimate age.
Moving inward from the bark, wood is classified into sapwood and heartwood. Sapwood is the outer, lighter band containing living cells, actively transporting water and storing food. Over time, inner layers of sapwood cease to function and convert into heartwood.
Heartwood accumulates secondary compounds (extractives), often giving it a darker color and higher resistance to decay, fungi, and insects. Although heartwood cells are dead, it provides structural strength. The overall direction of the wood, known as the grain, is established by the vertical alignment of the elongated cells, running parallel to the trunk axis.
Distinguishing Hardwood and Softwood
The classification of wood into hardwood and softwood is based on the tree’s botanical structure. Hardwoods come from flowering, broad-leafed trees, while softwoods originate from coniferous, cone-bearing trees. This distinction creates a fundamental difference in cellular architecture.
The most noticeable structural difference is the presence of vessels (pores) in hardwoods, specialized for water conduction. Hardwoods have a complex structure composed of vessel elements, fibers for support, and parenchyma cells. Vessel elements are much larger in diameter and are often visible as small pinholes on the end-grain surface.
Softwoods have a simpler, more uniform cellular structure, with over 90 percent of their volume consisting of longitudinal tracheids. These tracheids are multi-functional, performing both mechanical support and water transport. Since softwoods lack large vessels, they are anatomically referred to as nonporous woods, contributing to their consistent appearance.
How Structure Determines Physical Properties
The organization of wood directly determines its physical and mechanical properties. One consequence of cellular alignment is anisotropy, where strength and dimensional stability vary depending on the direction of the applied force. Wood is strongest when force is applied parallel to the grain, aligning with the long, supportive fibers.
Wood is weaker when loads are applied perpendicular to the grain, as resistance relies only on the weaker bonds between cells. This directional dependence results from the axial orientation of cellulose microfibrils within the cell walls. Understanding this difference is important for engineering applications, as wood is typically about 20 times stronger along its length than across its width.
The cellular structure also dictates wood’s response to moisture, known as hygroscopicity. This means wood readily absorbs and releases water vapor from the surrounding air. Moisture is held within the cell walls, and its amount affects all wood properties. The absorption of moisture causes the cell walls to swell, leading to dimensional changes in the wood.
These dimensional changes are also anisotropic; wood swells and shrinks differently in three structural directions: longitudinal (along the grain), radial (across the growth rings), and tangential (around the growth rings). Shrinkage is minimal longitudinally, typically around 0.4 percent from a green to an oven-dry state, because the fibers are long and stiff. However, tangential shrinkage is often about twice as great as radial shrinkage, which is why wood tends to cup and warp as its moisture content fluctuates.