Tree Biology

Single stemmed trees at RFS Leighton

What are trees?

The easiest definition is that a tree is a plant with a more or less permanent shoot system that is supported by a single trunk of wood.

Trees are the tallest free-standing organisms in the world. They live longer and grow larger than any other living organism on earth. There are more than 80,000 species growing worldwide and they come in all shapes and sizes; from tiny Arctic Willows a few centimetres high, through to giant Redwoods over 100 metres tall. The world’s tallest known living tree is a redwood tree called the Hyperion. It measures 115.61 metres (379.3 ft) in height, making it 22 metres taller than Big Ben’s tower.

Trees differ from shrubs. Shrubs normally grow near the ground and sometimes have several narrow stems rather than one single trunk. Shrubs can support many leaves because of their many stems. However, their overall structure is less rigid than one single, thick tree trunk.

What is the structure of a tree?

The cells from which all plants grow are called meristem. They are found at the tips of buds and roots. In most woody plants, there is also a layer of cells called the cambium. These wrap around every stem and limb of the plant like a glove. The cambium allows trees and shrubs to grow outwards as well as upwards in each growing season, allowing them to become much larger than plants without a cambium.

Inside a living tree trunk, there are several layers of cells.

The outer layer is the bark. Bark is tough and waterproof and protects the tree from the elements, insects, pests and fungal diseases. Bark also helps the tree to retain its moisture. As the trunk grows fatter, the bark spreads and cracks, often becoming gnarled in appearance.

Underneath the bark, is the phloem. This transports sugary sap made during photosynthesis from the leaves to all the other parts of the tree.

The next layer is the cambium which is responsible for the outward growth of the tree.

Beneath the cambium are several layers of woody tissue called xylem.

The first few layers of xylem are known as sapwood. They are responsible for transporting water from the roots to the rest of the tree.

The innermost layers of xylem are right in the centre of the tree. These layers are known as heartwood. The heartwood is dense and strong and provides stability as the tree grows.