The inner annual rings surrounding the central portion or core of a tree is _______.
Anatomy of a Tree Trunk
A cross-section of a tree trunk reveals several distinct layers, each with a specific function. Understanding these layers is fundamental to selecting and using timber correctly. The structure develops from the center outwards.
📝 Detailed Analysis of the Options
(a) Sap wood
Sapwood is the outer layer of wood in a growing tree, located between the heartwood and the cambium layer. It is the living, active part of the tree that transports water and nutrients. It is not the inner portion surrounding the core.
(b) Pith
The pith (or medulla) is the absolute central core of the tree trunk. It is the part that is surrounded by the inner annual rings, not the rings themselves.
(d) Medullary rays
These are thin, ribbon-like tissues that radiate from the center of the trunk outwards across the annual rings. Their function is to transport food and water horizontally. They are not the annular rings themselves.
(c) Heart wood
This is the correct answer. The heartwood is the inner, non-living portion of the trunk that surrounds the central pith. It is composed of the older, inner annual rings that have become inactive. As sapwood ages, it hardens and turns into heartwood, which is typically darker, denser, stronger, and more durable, making it the most valuable part of the timber for construction.
📊 Layers of a Tree Trunk
| Layer | Location | Description |
|---|---|---|
| Pith | Central core | The very center of the trunk. |
| Heartwood | Surrounds the pith | Inner, dead, strong, and durable annual rings. |
| Sapwood | Surrounds the heartwood | Outer, living, active annual rings that carry sap. |
| Medullary Rays | Radiate from the center | Thin tissues that run across the annual rings. |
💡 Study Tips
- Heart of the Trunk: Think of the heartwood as the structural "heart" or core of the tree, providing its main support.
- Pith is the Point: The pith is the starting point in the very center, not the rings around it.
- Sap Runs in Sapwood: This helps distinguish the living, outer sapwood from the dead, inner heartwood.