What is wood?
Wood comes from the trunk (main stem) of trees. A trees stem serves two main purposes:
to support the branches, leaves and lowers of the tree, holding these irmly, even against the buffeting of wind and storms.
to transport water and nutrients from the roots to the leaves, and sugar and other food stuffs from the leaves to all the other areas of the living tree. The cross-section of a tree trunk is made up of four principal layers. The outer-most section is a ring of bark made up of two layers: an outer layer of dead corky material, the outer bark, and an inner layer of live bark, the phloem. The outer layer is made up of epidermal cells that protect the stem from damage and from drying out.
The phloem contains cells which form tall and thin tubes, like capillaries, which transport the sugars and other materials made in the leaves to all the other living cells in the tree. The next layer is the cambium, which usually feels slimy in a freshly cut stem. This thin layer is made of cells which produce phloem and xylem, the next layer of the stem. The cambium is the only place in a stem where new growth takes place, and its cells are constantly dividing to form new wood and new bark. As a result of the continual division of cells, the cambium layer slowly moves outwards as the tree increases in girth. As the tree expands in girth, the outer bark periodically splits or is shed and is replaced by the new outer layer. The innermost layer of a stem is the xylem. Living xylem cells carry water and minerals from the roots to the leaves. Dead xylem cells make up heartwood which is the tissue (group of cells) in the centre of the stem
Different kinds of wood in a tree
Two kinds of wood are found in mature trees. The central part of cross-section of wood (usually the bulk of the cross-section) is the heartwood. Around the heartwood, in a broad ring, lies the sapwood. It is paler in colour compared to the heartwood and is often whitish or cream coloured.
Heartwood consists of dead material. It helps support the tree and has no role in the growth of the tree. Sapwood, on the other hand, is made up of living cells that carry water and nutrients upwards from the roots. It is this water and nutrient mixture that makes up a trees sap. New sapwood is formed by cambium cells as a tree grows. As new sapwood is formed, the innermost sapwood cells die and become heartwood. These cells slowly ill with tannins, resins and other substances, making the wood darker in colour and more resistant to decay and insect attack. The vessels that these cells form eventually become blocked and unable to carry sap
Annual rings
Trees that experience an annual growth pattern of slow and rapid growth rates are characterised by annual rings. each ring represents one years growth. Annual rings are found in most trees that come from europe and North America. By counting the number of annual rings, it is possible to determine a trees age. Some native trees do not have annual rings, though a few, like the snow gum, alpine ash and red cedar, do have visible annual rings. It is therefore dificult to readily tell the age of most Australian native trees. Growth rings are a feature of trees that grow in climates where growth virtually ceases for part of the year, such as during cold winter months. In spring, when these trees burst into growth, wood is formed relatively rapidly and these earlywood cells tend to be large and thin-walled. Later in the season, as tree growth slows, the cells become smaller and thicker-walled. The larger thin-walled cells tend to be paler in colour than the smaller thick-walled cells. An annual ring is made up of these two layers - the layers of thin- and thick-walled cell.
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