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Cross section

In a growing tree, the stem comprises the following main parts.

Outer Bark: This is the dead, corky, material that protects the stem from damage and stops the tree from drying out. As the tree grows in circumference, the bark gradually splits and falls off, and is replaced by new bark.

Phloem: The phloem forms the inner bark. It carries the food made in the leaves to all of the growing parts of the tree – that is, the branches, roots and stem.

Cambium: Underneath the phloem is a thin slimy layer of cambium. As its cells multiply, it forms new phloem tissue on the outside and new wood tissue on the inside. The cambium layer gradually moves outwards as the tree grows in girth.

Sapwood: The sapwood carries water and nutrients upwards from the roots. It is made up of living cells and is often lighter in colour than the heartwood. The sapwood and heartwood together are known as the ‘xylem’.

Heartwood: As new sapwood is formed by the cambium, some of the inner sapwood becomes inactive and is converted to heartwood. The dead cells are used to store waste products from the growing tree, so the vessels become blocked and are no longer able to carry sap. This often makes the heartwood turn a darker colour.

Pith: The small, soft core near the centre is called the pith. It is the original tissue in the tree from its early growth as a sapling.

Ray cells: The ray parenchyma cells run from the centre of the tree out to the bark. They are used to transport and store food supplies.

Diagram of cross section
Diagram of cross section