Structure of a Tree

From the visibility aspect, the structure can be divided into two categories:

1. Macrostructure
2. Microstructure

Macrostructure

The structure of wood visible to the eye or at a small magnification is called a macrostructure. The following are the different components (Figure 7.1).

1. Pith: The pith or medulla is the innermost part, seen only in old and immature trees. Wood of this zone is of black, brown or grey appearance. It is normally found in the first year of growth of the tree.
2. Heartwood: The inner annual rings surrounding the pith constitute the heartwood. It indicates the dead portion of the tree. It does not take active part in the growth of a tree. However, it imparts rigidity to the tree and, hence, forms durable timber for engineering purposes.
3. Sap wood: The sap wood comprises of new and lighter cells that line nearer to the skin of the tree. It indicates recent growth and contains sap. It takes active part in the growth of a tree and the sap moves in an upward direction through it.
4. Cambium: The thin layer of wood between the sap wood and the inner bark is known as cambium layer. If the bark is removed for any reason, the cambium layer gets exposed and results in the death of the tree.
5. Bark: It is the outermost zone and makes the skin of the tree. The function of the bark is to protect the inner tissue from heat, rain and injury. Sometimes a second thin membrane is also present inside the bark and it is called the inner bark.
6. Medullary rays: The thin radial fibres extending from the pith to the cambium is known as medullary rays. The function of it is to hold together the annual rings of hard wood and sap wood.

Figure 7.1 Cross section of an exogenous tree

Microstructure

The structure of the wood apparent only at great magnification is called microstructure. A living cell consists of four parts, namely membrane, protoplasm, sap and core. The cells according to their function are classified into conductive cells, mechanical and storage cells.

The conductive cells serve mainly to transmit nutrients from the roots to the branches and leaves. The mechanical cells impart strength to the wood and the storage cells serve to store and transmit nutrients to the living cells.