Trees are a familiar sight, shaping landscapes. While their outer appearance may seem straightforward, the internal structure of a tree trunk is remarkably intricate and organized. Each component plays a specific role in the tree’s survival and growth, revealing a complex biological system.
The True Name of Wood
The wood part of a tree is scientifically known as xylem. This specialized tissue forms the bulk of a tree’s trunk, extending from just inside the outer growth layers to the very center. Its primary function involves the upward transport of water and dissolved minerals from the roots to the leaves, a process driven by transpiration.
Xylem is composed of cells, many of which are dead at maturity, forming hollow, tube-like structures. These cells have thick, lignified cell walls. Lignin is a complex polymer that provides rigidity and strength, allowing the xylem to offer mechanical support to the entire tree, enabling it to stand tall against gravity and external forces. As xylem tissue matures, it becomes the wood we recognize, contributing to the increasing thickness and structural integrity of the trunk.
Layers Within the Wood
Within the xylem, two regions exist: sapwood and heartwood. Sapwood represents the younger, outermost portion of the xylem, typically lighter in color. It contains living cells and is actively involved in transporting water and nutrients throughout the tree. Sapwood also stores energy reserves and provides some mechanical support.
As a tree ages, the inner layers of sapwood cease their water-transporting function. These older xylem cells become heartwood, which is often darker in color and found at the core of the trunk. Heartwood consists of dead cells and primarily provides structural support and resistance to decay.
Understanding the Tree Trunk
While xylem constitutes the wood, a tree trunk comprises several other layers, each with a specific function. The outermost layer is the bark, which acts as a protective barrier against external threats like weather, insects, and disease. Beneath the outer bark lies the phloem, sometimes referred to as inner bark.
Phloem is a living tissue responsible for transporting sugars produced during photosynthesis from the leaves to other parts of the tree for growth and storage. Between the phloem and the xylem is a thin, actively growing layer called the vascular cambium. This cambium continuously produces new phloem cells towards the outside and new xylem cells towards the inside, facilitating the tree’s increase in girth and forming the annual growth rings seen in a cross-section of the trunk.