Growth and structure of exogenous tree

Exogenous trees (outward growers, such as sal, teak, pine) increase in diameter by adding new layers of wood each year under the bark. They provide almost all the structural timber.

Cross-section

         _______________
       /  ___________    \
      /  /  _______   \   \   Outer bark
     |  |  /  ___  \   |   |  Inner bark
     |  | |  ( o )  |  |   |  Cambium
     |  | |  pith   |  |   |  Sapwood
     |  |  \_______/  |   |  Heartwood
      \  \___________/   /   Annual rings
       \_______________/    Medullary rays

Macrostructure (components and functions)

1. Pith (medulla): soft central core; it is the first growth and has no strength.
2. Heartwood: dark, dense inner rings formed of dead cells; it gives strength and resists decay, so it is used as timber.
3. Sapwood: light, outer, living rings which carry sap from roots to leaves; weaker and less durable.
4. Cambium layer: thin layer of living cells between sapwood and the inner bark which forms new wood inward and new bark outward every year.
5. Annual rings: concentric rings of one year of growth each (spring wood is light and soft; autumn wood is dark and dense); counting rings gives the age of the tree.
6. Medullary rays: radial lines from the pith to the cambium which carry sap and store food, and bind the rings.
7. Inner bark (bast): carries food from leaves to the tree.
8. Outer bark: protects the tree from weather and insects.

Microstructure

Under the microscope, wood is made of elongated cells (fibres, tracheids and vessels in hardwoods) with hollow cavities, whose cell walls are mainly cellulose and lignin. Softwoods have tracheids (about 90% of the volume) that give strength and carry water; hardwoods have vessels (pores) for water, and fibres for strength.