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Understanding Tree Biology — Trunk and Branches


Main functions are protection and insulation. The bark consists of three layers of cell types: cork (phellem) on the outside, cork cambium (phellogen) in the middle and phelloderm on the inside.

Cork contains suberin, which is an important protective tissue — it provides protection for the tissues beneath it and prevents loss of water from the stem. Bark insulates internal tissues against extreme temperature fluctuations and is very resistant to attack by disease organisms and insects.


Its main function is transport of sugar from the leaves. A living tissue which is just beneath the bark and often interconnected with the outer bark. This is a complex, vascular tissue consisting of various living cell types. This sugar distribution system is like a living leaky pipe that allows sugars to move out to all living cells between the leaves and the ends of the roots.

Vascular Cambium

Main function is growth. These are meristematic cells which lead to an increase in girth of the tree by producing ‘xylem’ cells on the inside and phloem on the outside. It also produces ray cells. There are two types of cells in the cambium; one type differentiates into rays and the other type into phloem and xylem. This zone of growth regularly replaces all of the transport systems.

Xylem (Sapwood and Heartwood)

Xylem is known as wood — which is an orderly arrangement of living, dying and dead cells that have walls of mostly cellulose and lignin. Wood that contains living cells and transports water is called sapwood and wood without living cells and does not transport water is called heartwood.

Sapwood contains living cells and has four main functions: Transport of water and nutrients from the roots; Storage of energy reserves and other materials for maintaining life; Mechanical support; Protection and defence. Together with the phloem, the xylem forms a continuous system of vascular or transport tissue extending throughout the plant body. Like the phloem, sapwood is a ‘leaky’ pipe.

Heartwood contains non-living cells and their main function is mechanical support. The wood contains no cells with living contents and there is no active transport. Heartwood is often a different colour to the sapwood because, as the nutrients leave the cells, the cells die, the cell walls are altered and chemicals such as gums, oils, resin and tannin may impregnate and accumulate. It does maintain a protection and defence system and continues to provide mechanical support.

The heartwood provides the best structural timbers.

Growth Rings

Secondary growth of trees is produced by the vascular cambium. The seasonal activity of cambium produces growth increments or growth rings.

Sudden changes in available water, other environmental factors, or damage to the tree may be responsible for disrupting growth rings. The width of a growth ring is a good indicator of the prevailing rainfall of the time and can also be used to date a stress in the life of the plant.

The reason for being able to see the rings in wood is the difference in density of the wood cells produced during the growing season.

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