What is wood?
From the Autumn/Winter 2009 edition of Bush Telegraph Magazine.
Wood comes from the trunk (main stem) of trees. A tree’s stemserves two main purposes:
- to support the branches, leaves and flowers of the tree
- to transport water and nutrients from the roots to the leaves, and sugar andother 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 fourprincipal layers. The outer-most section is a ring of bark made up of twolayers: an outer layer of dead corky material, the outer bark, and an innerlayer of live bark, the phloem.
The outer layer is made up of epidermal cells that protectthe 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 theleaves to all the other living cells in the tree.
The next layer is the cambium, which usually feels slimy in afreshly cut stem. This thin layer is made of cells which produce phloem andxylem, the next layer of the stem.
The cambium is the only place in a stem where new growthtakes place, and its cells are constantly dividing to form new wood and newbark. As a result of the continual division of cells, the cambium layer slowlymoves 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 outerlayer.
The innermost layer of a stem is the xylem. Living xylemcells carry water and minerals from the roots to the leaves. Dead xylem cellsmake up heartwood which is the tissue (group of cells) in the centre of thestem.
Different kinds of wood in a tree
Two kinds of wood are found in mature trees. The central partof cross-section of wood, usually the bulk of the cross-section, is theheartwood. Around the heartwood, in a broad ring, lies the sapwood. It is palerin 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 rolein the growth of the tree.
Sapwood, on the other hand, is made up of living cells thatcarry water and nutrients upwards from the roots. It is this water and nutrientmixture that makes up a tree’s sap.
New sapwood is formed by cambium cells as a tree grows. Asnew sapwood is formed the inner-most sapwood cells die and become heartwood.These cells slowly fill with tannins, resins and other substances, making thewood darker in colour and more resistant to decay and insect attack. Thevessels that these cells form eventually become blocked and unable to carrysap.
