In This article Learn growth of timber,timber,sap wood,annual rings,cambium layer,uses of timber,classification of trees,growth of timber tree and its structure.



Wood suitable for building or other engineering purposes is called timber. When it forms part of a living tree it is called standing timber. When the tree has been felled it is called rough timber. When it has been sawn to various market forms such as beams, battens and planks etc., it is called converted timber.


Timber is used for the following categories of works :

(i)        For construction purposes, including building construction, houseposts, beams, rafters, bridges, piles, poles and railway sleepers etc.

(ii)       For furniture and cabinet making.

(iii)     For light packing cases.

(iv)      For heavy packing cases (for machinery and similar stores).

(v)       For manufacturing agricultural implements and tool handles.

(vi)      For making turnery articles and toys etc.

(vii)    For manufacturing veneers and plywoods.


Depending on the mode of growth trees are classified into two categories as : (a) Endogenous, and (b) Exogenous

1. Endogenous

trees are the ones that grow inwards in a longitudinal fibrous mass such as banana, bamboo, plam and cane. Even though the “stem” of trees of this class is light and tough yet it is too flexible and slender to furnish material suitable for engineering works, with the exception of bamboo.

2. Exogenous

trees are those that grow outwards by the addition of one concentric ring every year. These rings are known as annual rings. Since one ring in the stem of a tree indicates its age in years. It is timber obtained from this class of trees that is extensively used in engineering works.

Timber available from exogenous trees is further classified into two categories as :

(a)       Conifers or evergreens yielding Soft wood. These are trees with pointed leaves. Deodar, Pine, chir and kail belong to this class.

(b)       Deciduous are trees with broad leaf, yielding Hard wood. Teak, sal, shisham belong to this class.

This classification is based on different properties of the wood available from different trees.


 1.  Growth.

In spring season roots of the tree suck a solution of salts from the soil-salts that are food for the tree and transmit the same through the trunk of tree to its branches and leaves. This solution of salts looses some of the moisture because of evaporation and absorbs carbon dioxide from the air. This action in the presence of sun makes the solution a bit viscous. This transformed viscous solution is known as sap.

In autumn viscous sap descends below the bark and leaves a thick layer. Layer of sap left below the bark gets transformed to wood and is known as cambium layer. It goes on gaining strength with the passage of time. A fresh layer is thus added on the outside of the tree every year forming a new annual ring. The new ring represents a year’s growth of tree.

Medullary rays carry the sap from below the bark to the interior thereby nourishing the tree.

2. Structure.

On examining the cross section of the trunk of an exogenous tree, we see the different parts as shown in Fig. 9.1. A brief description of each part is discussed separately.

Structure of exogenous tree Growth. Structure. Pith or medulla. Annual rings. Heart wood. Sap wood. Cambium layer. Medullary rays. Bark.

3. Pith or medulla.

It is the first formed portion of the stem of tree. It consists entirely of cellular tissues. The pith, which when the plant is young, containis a large amount of fluid and nourishes the plant. It dies up and decays when the plant becomes old. Sap is then transmitted by the woody fibres that deposit about the pith. Pith of branches is a mere prolongation of the pith of stem.

4. Annual rings.

The rings of woody fibre arranged in concentric circle around the pith are known as annual rings because one such ring is added every year.

5. Heart wood.

Innermost rings surrounding the pith constitute the heart wood. This wood is darker in colour, stronger, more compact and durable.

6. Sap wood.

Outer annual rings of the tree constitute the sap wood which transmits the sap from roots to branches. Compared with heart wood, sap wood is lighter in colour, weaker and more liable to decay.

7. Cambium layer.

Outermost one ring between the bark and sap wood which is not yet converted into wood is known as the cambium layer. In due course, cambium layer changes to sapwood. If the cambium layer is exposed by removing the bark, the cells cease to be active and results in death of tree.

8. Medullary rays.

These are thin horizontal veins radiating from the pith towards the bark. They carry sap from outside to the inner parts of tree and nourish it. They keep the annual rings tightly gripped together. In some trees they might be found broken or may not even be clearly visible.

9. Bark.

It is outermost protective covering of cells and woody fibres on a tree. In course of time older layer split and scale off.