Structure of Timber |Macrostructure and Microstructure

The structure of timber essentially describes the parts of timber (wood) internally at macro and micro levels. 

As we know, timber can be obtained from exogenous and endogenous trees. Exogenous trees are most used in structural applications and have more visibility of internal structure. Hence we use the cross section of an exogenous tree to describe the component part of the timber layer.

From the Visibility Aspect, the structure of a timber or tree can be divided into two categories:
  1. Macrostructure
  2. Microstructure

1. Macrostructure of Timber

The structural components of timber that are visible to the eye or at a small magnification are called macrostructure. The cross-section of a tree has several components which differ from one tree to another. The layers include the pith or medulla, heartwood, sapwood, cambium layer, medullary rays, and bark.

The different elements constituting the structure of a tree are explained briefly below:

Structure of Timber | Building Materials

1. Pith or Medulla

The innermost central portion that contains the entire cellular tissue is called the pith or medulla. Pith is responsible for the nourishment of the tree in its younger age. 

2. Sapwood and Heartwood

When you observe the cross-section of an exogenous tree, there are annual rings surrounding the pith. As shown in Figure 2, there are inner annual rings that are dark in color and outer annular rings that are light in color. 

Sapwood or Alburnum

Sapwood are a few outer annual rings that are light in color and near to the skin of the tree. Sapwood plays an important role in taking nutrients from the roots to the leaves, storing them and supporting the tree. It contains "sap" and sapwood is responsible for the nourishment of the tree in its living agent or recent growth.   

Sapwood and Heartwood


The inner annual rings surrounding the pith constitute heartwood. Essentially heartwood is the sapwood that has been clogged with resin and got harder to support a tree is called hardwood. Hence, the annual rings that surround the pith are called heartwood. 

Heartwood is dark in colour and does not take part in the growth of a tree. Hence it is called the dead portion of the tree.  This part forms the strongest and most durable part of a tree. 

Heartwood is responsible for imparting rigidity to the tree and hence a tree with good heartwood yields durable timber for engineering applications.

Difference Between Sapwood and Hardwood

MeaningRefers to the outermost layer of wood, which is soft in natureRefers to the dense central region of wood, which is hard in nature
Alt. NamesAlburaDuramen
LocationFound in the peripheral regionFound in the central region
CellsContains living cellsContains dead cells
ColorLighter in colorDarker in color
CompositionHigher in cellulose contentHigher in lignin content
ConductivityActs as a conductorActs as an insulator
FunctionFacilitates transport of water and nutrientsProvides structural support
DurabilityLess durable due to attack by pathogensQuite durable due to resistance to pathogen attacks

3. Annual Rings or Growth Rings

Annual rings are concentric circular rings surrounding the pith. Each ring signifies one year of growth of the tree. As shown in  the figure below, the cross-section show 24 distinct annual rings. It is a combination of heartwood and sapwood. 

4. Cambium Layer

The thin sap layer around the sapwood, (between the bark and sapwood) is termed as cambium layer. This layer contains sap which is yet to be converted into sapwood or it is the reproductive layer that results in new tissue formation.

5. Inner Bark

The layer surrounding and protecting the cambium layer is called the Inner bark.  

Outer and Inner Bark of a Timber

6. Outer Bark

The outer skin which is the protective layer of the tree is called bark or cortex. As shown in the figure above, a bark insulates the tree against temperature exposure and saves the sapwood and phloem from drying out.

7. Phloem

A phloem is a thin spongy layer that serves the function of carrying dissolved sugar and growth hormones from the leaves to other parts of a tree, as shown in the figure above.

Phloem and Medullary Rays

7. Medullary Rays

These are vertical layers of cellular tissues and are thin radial lines from the plinth to the cambium layer. The function of medullary rays is to hold the annual rings together.

Microstructure of Timber

The structure of wood that is visible only by a greater magnification is called microstructure. A living cell consists of a membrane, protoplasm, sap, and core. Based on the function of a cell, they are classified into: 
  • Mechanical Cell: Provide strength to the timber
  • Conductive Cell: Conduct or transport nutrients from roots to branches and leaves
  • Storage Cell: Stores and transmits the nutrients

Post a Comment


  1. Anonymous19 December

    This is very nice for Basic Technology students

  2. Anonymous15 June

    This is nice

  3. Anonymous25 September

    But hard to draw


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