Wood is a sophisticated biological material produced by trees as secondary xylem. This tissue transports water and nutrients upward while providing the mechanical support necessary for the tree to stand against gravity and wind. The strength and workability of wood have made it a globally significant resource for construction, fuel, and countless products. To understand its properties, one must examine its chemical composition and cellular organization.
The Major Chemical Building Blocks
The primary structural component of wood is cellulose, which typically accounts for 40 to 50 percent of the wood’s dry weight. This long-chain polysaccharide is organized into microfibrils, acting much like the reinforcing steel rods within concrete. These microfibrils provide tremendous tensile strength, allowing the wood to resist being pulled apart along the grain.
Providing rigidity and compressive strength is lignin, a complex, amorphous polymer that makes up 20 to 35 percent of the wood mass. Lignin functions as the matrix, encasing the cellulose microfibrils and binding them together. This rigid binder also provides protection against biological degradation and makes the wood largely impermeable to water.
Hemicellulose is the third major polymer, a group of shorter, branched polysaccharides that typically constitute 15 to 25 percent of the wood. Its role is to stabilize the structure, acting as a filler that links the cellulose and lignin components together. The remaining wood mass consists of water and various organic extractives, such as resins, tannins, oils, and pigments.
The Cellular Architecture of Wood
Wood is composed of millions of specialized, elongated cells cemented together by the lignin matrix. These cells, remnants of the tree’s vascular system, are aligned vertically, parallel to the trunk’s axis. This parallel orientation defines the wood’s grain, which dictates how the material splits, bends, and absorbs finish.
The process of growth results in distinct annual rings, marking one year of the tree’s life and defined by two types of wood. Earlywood, formed during the rapid growth of spring, consists of larger, thinner-walled cells designed for efficient water transport. Latewood, formed in the slower summer months, features smaller, thicker-walled cells that provide greater structural density and strength.
Moving inward from the bark, one encounters the sapwood, the living, metabolically active outer layer of the trunk. Sapwood conducts water and stores carbohydrates for the tree. Over time, the inner cells cease to function in transport and undergo chemical changes, transitioning into the heartwood.
The heartwood is metabolically inactive and acts purely as the central structural column of the tree. During this transition, cells become impregnated with various extractives like tannins and resins, which often darken the wood’s color and increase its resistance to decay. This change explains why heartwood is typically denser and more durable than the surrounding sapwood.
Structural Differences Between Hardwoods and Softwoods
Wood is broadly categorized into two types based on the trees they originate from: hardwoods (from angiosperms, or flowering trees) and softwoods (from gymnosperms, or cone-bearing trees). This classification is based on botanical origin, not necessarily the actual physical hardness of the wood. The primary difference lies in the specialized cell types used for water conduction.
Hardwoods possess specialized, large-diameter conducting cells known as vessels or pores, which are visible as tiny holes on the end-grain surface. These vessels are highly efficient for water transport but are intermixed with fiber cells that provide the bulk of the mechanical strength. The arrangement of these vessels—whether diffuse or ring-porous—significantly influences the wood’s visible texture and overall density.
Softwoods, which typically represent the majority of commercial lumber, have a simpler and more uniform cellular structure. They lack true vessels and instead rely on long, thin cells called tracheids to perform both water transport and mechanical support. The consistent structure of the tracheids results in a much less pronounced grain pattern compared to most hardwoods.