The structure of timber, or wood, is characterized by several distinct layers and components that contribute to its overall strength and properties.
Layers of a Tree Trunk:
The structure of timber primarily refers to the arrangement of tissues within a tree trunk, especially as they are observed in sawn timber. Here's a breakdown of the key components:
-
Pith: The pith is the soft, central core of the tree trunk. It's composed of spongy tissue and is usually the weakest part of the timber.
-
Heartwood: Surrounding the pith is the heartwood. This is the older, inactive wood, often darker in color due to the deposition of extractives (resins, gums, tannins). Heartwood provides structural support to the tree and is generally more resistant to decay and insect attack than sapwood.
-
Sapwood: The sapwood is the outer layer of wood surrounding the heartwood. It's the living part of the wood responsible for transporting water and nutrients from the roots to the leaves. Sapwood is typically lighter in color than heartwood and more susceptible to decay and insect infestation.
A Visual Representation
While not easily represented in a simple table, think of the timber structure as concentric rings. The innermost is the pith, followed by the heartwood, and finally the sapwood as the outermost ring before the bark.
Cell Structure
Beyond the macroscopic layers, the microscopic structure of wood is also critical. Wood is composed of cells, primarily:
- Tracheids (in softwoods): These long, slender cells provide both support and water transport.
- Vessels (in hardwoods): Larger diameter cells that primarily function in water transport.
- Fibers (in hardwoods): Provide structural support.
- Parenchyma cells: Involved in food storage.
The arrangement and characteristics of these cells (e.g., cell wall thickness, density) significantly affect the properties of the timber.
Grain
The term "grain" refers to the direction of the wood fibers. Straight-grained timber is easier to work with and is generally stronger than timber with irregular or spiral grain.
Conclusion
Understanding the structure of timber, from its macroscopic layers (pith, heartwood, sapwood) to its microscopic cell structure and grain orientation, is essential for selecting and using timber effectively in construction and other applications.
