A tree is a perennial woody plant that builds a strong, upright structure to compete for sunlight. This massive form requires a sophisticated combination of materials and biological systems working in concert. To understand what a tree is made of, one must look beyond the visible trunk to its microscopic chemical composition and the layered architecture that allows it to grow tall and strong.
The Primary Chemical Building Blocks
The physical strength of wood comes from three primary organic polymers that form a natural composite material. Cellulose provides the main fibrous structure, acting like reinforcing rods. It is the most abundant organic polymer on Earth, forming long, linear chains of sugar molecules that bundle together into microfibrils with high tensile strength.
Lignin is a complex polymer that acts as the rigid binder or “glue,” filling the spaces between the cellulose fibers. This polymer provides compressive strength and stiffness, allowing the tree to withstand the enormous pressure of its own weight and resist wind forces. Lignin also imparts a measure of water resistance and protection against decay.
Hemicelluloses are shorter, more branched polysaccharide molecules that link the cellulose and lignin together in the cell wall structure. They act as a kind of matrix or filler, contributing to the overall integrity of the wood. They play a role in the nanoscale architecture that enhances the material’s mechanical properties.
The Living Transport and Growth Systems
Just beneath the rough, protective surface is a set of living tissues responsible for moving nutrients and generating new growth. The outermost layer is the bark, which provides a physical shield against insects, disease, and mechanical damage. It also insulates the delicate internal tissues from temperature extremes and is continually renewed from within as the tree grows.
The phloem, often called the inner bark, is the tree’s pipeline for transporting sugars created during photosynthesis. This living tissue moves the dissolved sugars downward to fuel growth and storage in the roots and other non-photosynthetic parts of the tree. As new phloem is produced, the older cells die and are crushed outward to become part of the protective outer bark.
Positioned between the phloem and the wood is the vascular cambium, a thin, active layer of dividing cells. This is the growth engine that allows the tree to increase in diameter each year. The cambium produces new phloem cells toward the outside and a much larger quantity of new xylem cells toward the inside.
The Structural Foundation of Wood
The bulk of the tree’s trunk is made up of xylem tissue, which is the wood itself. Xylem cells are responsible for transporting water and dissolved minerals upward from the roots to the leaves. As these specialized cells mature, their walls are heavily reinforced with lignin before the cells die, leaving behind a network of hollow, interconnected tubes that form the strong, supporting structure.
The living, outer layers of xylem are known as sapwood, which is typically lighter in color and contains the active water-conducting cells. Sapwood is a metabolically active region, also functioning to store carbohydrates and provide defense against pathogens. The number of sapwood rings remains relatively constant, as the tree only needs a certain volume of active tissue to meet its water requirements.
As the tree ages, the innermost layers of sapwood cease to conduct water and undergo a chemical transformation to become heartwood. This process involves the depositing of chemical compounds, called extractives, such as resins and phenols, into the dead xylem cells. The heartwood is often darker, denser, and more resistant to decay and insects, serving primarily as the central, non-living column of structural support for the entire tree.
The seasonal production of xylem by the cambium creates the visible growth rings. In the spring, when water is plentiful, the cambium produces large cells with thin walls, known as earlywood, which appear lighter. As the season progresses, growth slows, and the cells produced, called latewood, are smaller and have thicker walls, resulting in the darker band that marks the end of the year’s growth.