In addition to acting as the primary strengthening and nutrient conducting tissue of trees and other plants, wood is extremely versatile in its applications for human use. The abundant natural material is utilized in many common products, such as lumber, furniture, and paper, but is perhaps most important as the primary source of fuel in many parts of the world.

View a second image of a wood radial section.

Trees have a natural lifespan, but may live for hundreds and even thousands of years. Once trees reach maturity, which differs by species, growth decelerates and decay begins, eventually resulting in death. The age of a tree can be estimated by the number of growth rings present in a transverse section of wood. In temperate regions, one concentric layer is usually formed during each growth season, but in the tropics the development of the rings is more variable, and thus the age of tropical trees is more difficult to ascertain.

Wood is composed of several types of cells, including tracheids, vessel members, fibers, and parenchyma. Whether a tree is considered a softwood or hardwood species is based on the cellular composition of its wood. Softwood species are predominantly composed of tracheids, but also contain parenchyma. Hardwoods, however, are made of vessel members, fibers, parenchyma, and, occasionally, tracheids.