Unit 1 - Structural Units & Functions

Lesson 7: Trunks and Branches

All trees have a trunk and branches. The trunk is the main stem of the tree; it supports the branches which in turn support the leaves.

The outermost layer of the trunk and branches is called the outer bark. It protects the tree against disease, insects, fire, drought, injury, and animals.
Right underneath this layer of protection is an inner bark layer known as the phloem. The primary function of the phloem is to carry manufactured food (sugars) from the leaves to all living parts of the tree, including the roots. As phloem cells die, they become part of the bark found on the outside of the tree.
Directly beneath the phloem is a layer known as the cambium. The cambium is a thin layer of cells, which produces new phloem cells to the outside and new xylem cells to the inside. Xylem cells give rise to what is known as the tree's sapwood. The sapwood is responsible for the transport of water and minerals inside the tree from the roots to the leaves.

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As the xylem cells of the sapwood die, they become part of the heartwood or centre of the tree. The heartwood gives the tree strength to stand.

If you look at a cross-section of a tree stump or piece of wood, you can determine the age of that wood. There is a growth line for every winter the tree has survived. The lighter areas in the heartwood represent the area of growth for each year.