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Wood's importance

Wood is one of civilisation's earliest and most basic materials. One of humanities first studies was to determine the properties of the wood from different trees. To be able to select the most suitable timber for your requirements, an understanding of the structure and basic properties of wood is essential. Wood's features, tree growth, and wood formation must be understood. The differences in wood structure effecting its material characteristics and function can then be investigated. The physical and mechanical properties of wood is related to its cell wall properties. It must also be realised that the physical and mechanical properties of wood are not fixed - they fall within boundaries or ranges - because wood is a variable material [1].

Types: Hardwood and softwood

There are two main types of natural timber - hardwood and softwood.

The categories softwood and hardwood do not describe the properties of the timber but refer to the types of tree from which the woods are obtained.

Softwoods come from trees that possess seed-bearing cones - known as coniferous trees. Coniferous trees are evergreens - they keep their leaves and grow all year round - and usually have needle-like leaves. Many softwood trees take between 25 and 50 yrs to reach maturity - although some have been known to be up to 4800 years old (the Great Basin bristlecone pine in the USA). They tend to be less expensive than hardwoods. Coniferous trees are adapted to a wider range of climates than hardwoods - they occur naturally from north of the artic circle in Finland right down to the Equator in Sumatra - and usually can be found at higher altitudes than hardwoods. In Australia, native conifers can be found from north Queensland to Tasmania. (See the section on Softwoods for more details).

Hardwoods come from broadleaved, flowering trees (dicotyledonous angiosperms). The term has no reference to the actual hardness of the wood. Angiosperms produce a fruit or nut. Hardwood trees that are used commercially for timber generally take longer than conifers to reach maturity - up to 100 yrs or more. They grow in warmer climates and tend to be more expensive than softwoods. (See the section on Hardwoods for more details).

Wood anatomy

Softwoods are usually relatively uniform in their appearance because 90% of softwood wood is composed of a single cell type - the longitudinal tracheid. The tracheid's function is to support the tree and to allow water conduction to the leaves. The other 10% of softwoods are composed of ray parenchyma. (Rays usually are only 1 cell wide). Some softwoods also have small amounts of other cells. In softwoods, the tracheid cell has two roles [1].

Follow this link to see views of the longitudinal tracheid [1]. The photographs are taken with both a scanning electron microscope and hand-lens ( large 183 kb image file).

Follow this link to see how tracheids look from different sections through the log. When you arrive at this hyperlink, click on "Radial view" (55 kb image file size) then "Tangential view" (62 kb image file size) [1].

The distinguishing feature of hardwoods is the vessel element, an open-ended cell. Hardwoods have longitudinal "vessels" that transport solutions, "fibers" that perform the mechanical support role, and "parenchyma" for storage. Fibres make up the bulk of the wood. Hardwoods, being 'vessel-permeated', therefore have a more complex appearance than softwoods. In hardwoods, each type of cell has a specialist role [1].

Follow this link to see vessel elements in hardwood [1]. When you follow this hyperlink, scroll down the webpage to see the photographs.

General wood properties

Although the wood of all trees consists essentially of cellulose fibres held together by lignin, different trees produce woods with many different properties that make them suitable for different purposes.

In the important section on general wood properties the three distinct surfaces of wood will be explained: cross section, radial, and tangential. Wood not only looks different in cross section, radial, and tangential views, but it also has considerably different properties depending upon its orientation.

Manufactured boards

Both hardwoods and softwoods can be formed into manufactured boards (plywood, hardboard, MDF, chipboard). Manufactured timber is processed by cutting up solid timber and putting it back together in a variety of different ways. The main reason for doing this is to end up with large flat sheets of timber which are stronger, more stable and less expensive than comparible sized timber. When a tree is milled (cut up into usable sized pieces - like the ones you use in the wood-shop), up to 40% is left as waste (sawdust and small odd-sized pieces) so this waste material is manufactured into wood product sheets. (See the section on Manfactured boards for more details).

Glossary

angiosperm: Flowering plants (seeds enclosed in an ovary - flowering plant).

dicotyledonous: A flowering plant with two embryonic seed leaves or cotyledons that usually appear at germination.

fibers: (Botany). One of the elongated, thick-walled cells that give strength and support to plant tissue.

parenchyma: Thin-walled cells that remain capable of cell division even when mature, constitutes the greater part of leaves, roots, the pulp of fruits, and the pith of stems.

vessels: (Botany). One of the tubular conductive structures of xylem, consisting of dead cylindrical cells that are attached end to end and connected by perforations. They are found in nearly all flowering plants.

xylem: The supporting and water-conducting tissue of vascular plants, consisting primarily of tracheids and vessels; woody tissue.

Citation

[1] Wheeler, Dr. E.A. (1997, June). North Carolina State University: Wood Anatomy & Properties [WWW Document] URL http://courses.ncsu.edu/classes/wps202001/202syl/wps202.html (visited January, 2001).

Copyright D. L. Christiansen [Last updated February 2001] Images: respective copyright owners noted/cited.

 

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