The interior of a tree is a highly organized, layered biological structure. The trunk functions like a complex system where each concentric layer has a specific name and a distinct job to keep the organism growing and alive. These layers transition from protective armor on the outside to active growth and finally to a supportive core. Understanding these layers reveals how a tree stands tall and transports nutrients.
The Protective Exterior and Nutrient Flow (Bark and Phloem)
The outermost layer of a tree is the bark, which serves as a shield against the outside world. This layer is composed of dead tissue, continually renewed from within, acting as armor against weather, temperature fluctuations, and physical damage from insects or animals. The thickness and texture of the bark vary between species and age, providing insulation and helping to prevent moisture loss.
Just beneath the outer bark lies the phloem, a living vascular tissue. The phloem acts as the tree’s food distribution system, moving sugars produced during photosynthesis in the leaves downward. This sugar-rich solution, called sap, travels to the roots and other non-photosynthesizing parts to fuel growth and storage. Phloem cells have a short lifespan and, once they die, are pushed outward to become part of the outer bark.
The Growth Engine and Water Channels (Cambium and Sapwood)
The vascular cambium is a layer of actively dividing cells situated between the phloem and the next layer of wood. This layer is responsible for the tree’s secondary growth, which is the increase in its girth or diameter. The cambium cells divide to produce new phloem cells on the outside and new wood cells, or xylem, on the inside.
The wood cells formed by the cambium constitute the sapwood, the active layer of the tree’s water transport system. Sapwood is composed of living and dying xylem cells that form a network of vessels running vertically up the trunk. This network moves water and dissolved minerals from the roots up to the leaves. The distinct annual rings visible in a cross-section of a tree are formed by the xylem cells produced by the cambium layer each growing season.
The Central Core Structure and Support (Heartwood and Pith)
As the tree grows and the cambium produces new rings of sapwood, the older, inner xylem cells lose their function of water transport. These non-functional cells compact and harden to become the heartwood, which forms the dense, central core of the trunk. Heartwood is essentially dead tissue, but it provides the structural strength and rigidity necessary to support the tree’s height and withstand external forces.
Heartwood often appears darker than the surrounding sapwood because the tree deposits compounds like tannins, resins, and oils into these dead cells. These compounds increase the wood’s density and provide resistance to decay, fungi, and insects. At the center of the heartwood is the pith, a small, soft core of cells that was the primary tissue for storage and transport when the tree was a young seedling.