The material that forms the bulk of the body of trees is called wood. Wood is the major product of the metabolism of trees and because of its many unique properties it is a very essential and useful material for the human being. Wood has been in use for human needs from time immemorial. Even today wood is largely used for making tools, machinery, furniture and house building. It also serves as food material for many fungi and wood-eating animals such as termites.
Fig: Relative Positions of Vascular Tissues
The many properties of wood include the following: it is light, a poor conductor of heat, electricity and sound; easy to work with; holds paint, lacquer, varnish and other finishing materials because of its porous nature; strong and absorbs shock; bears load;, does not rust; can be easily converted to pulp and is beautiful and appealing.
Wood is formed by vascular cambium during secondary growth when it gives rise to secondary xylem. The cambium of each vascular bundle is called fascicular cambium. First of all, the cells of the medullary rays in line with those of the fascicular cambium become meristematic and form strips of interfascicular cambium. As these strips join with the strips of fascicular cambium, a complete ring of cambia is formed. The cambium has two types of cells namely fusiform initials which are elongated and produce fibres, sieve tubes, tracheids and other vascular elements and ray initials which produce parenchymatous cells of the rays in wood and phloem.
Annual and Growth Rings
The cambial activity is greatly influenced by climatic conditions. In winter cambial activity is very low and the secondary tissues have fewer elements with narrower lumens. The secondary xylem produced in winter is called winter wood and that produced in spring is called spring wood. In a transverse section of a stem, these two types of wood appear in the form of distinct concentric circles known as growth rings. A spring wood circle together with a winter wood circle constitutes an annual ring.
Fig: Diagrammatic Section through a Woody Stem
Another feature which adds to the appearance of wood is the difference between old and new regions. As a tree grows in diameter, the xylem towards the centre becomes clogged with resins and stops conducting water and minerals. This is the heart wood which is darker and shinier in appearance. The sap wood is that portion which is responsible for all water and mineral conduction in the tree. This portion of the wood is lighter and more porous. Knots, which may be considered attractive as in knotty pine, are a major defect in structural timbers as they tend to weaken the wood. Knots are formed as a result of branching of the main trunk.
The possible limitations of such a versatile material as wood are its mechanical and physical properties. They cannot be altered or improved by heating. Being an organic material it is subject to infection by micro-organisms and it is combustible.