The classification of timber is depending upon its mode of growth trees may be divided into the following categories:
1)Exogenous trees:
These trees increase in bulk by growing outwards and distinct consecutive rings are formed in the horizontal section of such a tree. These rings are known as the annual rings because one such ring is added every year and these rings are useful in predicting the age of the tree. Timbers from exogenous trees only are used for engineering works.
Exogenous trees are further subdivided into:
a) Conifers and
b) Deciduous
a) Conifers:
These are evergreen trees having pointed, needle-like, or scale-like leaves. These trees yield softwoods. Examples: deodar, pine, etc.
b) Deciduous:
These trees are also known as the broad-leaf trees and the leaves of these trees fall in autumn and new ones appear in the spring season. These trees yield hardwood. Examples: babul, mahogany, oak, sal, teak, etc.
Softwoods:
The softwoods form a group of evergreen trees. The cellular structure of softwoods is simple and shows fairly distinct annular rings. They have straight grains, uniform texture, and light. color. They are resinous and are light in weight.
Hardwoods:
The hardwoods form a group of broad-leaf trees. Hardwoods have a complex cellular structure and annular rings are not so distinct. They are non-resinous, usually dark in color and heavy in weight.
2)Endogenous trees:
These trees grow inwards and fibrous mass is seen in their longitudinal sections. The timber from these trees has very limited engineering applications. Examples of endogenous trees are bamboo, cane, palm, etc.
STRUCTURE OF A TREE
A tree basically consists of three parts, namely, trunk, crown, and roots.
The structure of a tree can be divided into two categories:
a) Macrostructure
b) Microstructure
a) Macrostructure
The structure of wood visible to the naked eye or at a small magnification is called the macrostructure. The cross-section of a tree. The important parts are as follows:
Pith:
The innermost central portion or core of the tree is called the pith or medulla.
Annular rings:
These are the concentric circles or rings of woody fiber around the pith. Exogenous trees increase in bulk by growing outwards and one ring is added every year and these rings are useful in predicting the age of the tree.
Heartwood:
Heartwood consists of innermost annual rings. It is usually dark in color. It is considered to be the most strong, durable, and a compact portion of the tree. It provides strong and durable timber for various engineering purposes.
Sapwood:
The portion of the timber enclosed between the heartwood and cambium layer is known as sapwood. Sapwood is light in color than heartwood. It indicates recent growth and it contains sap.
Cambium layer:
The thin layer of cells between sapwood and the inner bark is called the cambium layer. It indicates sap that has yet not been converted into sapwood.
Inner bark:
The inner skin or layer covering the cambium layer is known as the inner bark. It gives protection to the cambium layer from any injury.
Outer bark:
The outermost cover or skin of the tree is known as the outer bark.
Medullary rays:
The thin radial fibers extending from the pith to the cambium layer are known as the medullary rays. These rays help to hold the annual rings together.
b) Microstructure:
The structure of wood apparent only at great magnifications is called the microstructure. when studied under a microscope, it becomes evident that wood consists of living and dead cells of various sizes and shapes.
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