In biology, plants grow by adding layers of tissue to their trunks each year, creating visible patterns known as annual rings.

What are Annual Rings?

Trees in temperate regions grow differently depending on the season. During spring, the tree creates large, light-colored cells because there is plenty of water. In autumn, the tree grows more slowly, creating small, dark-colored cells. This contrast between light and dark wood forms a ring. One light ring plus one dark ring equals one year of growth. Scientists can count these rings to determine the age of a tree, a process called dendrochronology.

Heartwood: The Sturdy Core

As a tree grows older, the oldest layers of wood in the center stop carrying water. These cells die and become filled with resins, gums, and tannins. This central, hard, and often dark part of the tree trunk is called heartwood. It no longer helps in transporting water, but it provides incredible strength and structural support to the tree. Because of the chemicals stored inside, heartwood is naturally resistant to decay and insect attacks, making it highly valued for furniture making.

Sapwood: The Living Transport System

Surrounding the heartwood is a lighter, softer, and more active layer of wood called sapwood. This is the living part of the tree's xylem, which is the tissue responsible for moving water and minerals from the roots up to the leaves. Unlike heartwood, the cells in the sapwood are alive and functioning. If you look at a cross-section of a tree, the sapwood is usually the pale ring located just beneath the bark. As the tree gets wider, the inner layers of the sapwood eventually die and turn into heartwood.

Summary Comparison

• Annual Rings: Visible circles showing the tree's age.
• Heartwood: The dark, dead, inner part that provides strength.
• Sapwood: The light, living, outer part that transports water.