The structure of wood that is visible to the eye or seen on a small scale is called macrostructure. Following are the different components.
Pith: The pith or medulla is the innermost part, seen only in old and immature trees. The wood of this zone is black, brown or gray in color. It is usually found in the first year of the tree’s growth.
Heartwood: The inner annual rings around the pit form the heartwood. It indicates the dead part of the tree. It does not take an active part in the growth of the tree. However, it imparts hardness to the tree and hence makes a durable wood for engineering purposes.
Sapwood: Sapwood consists of new and lighter cells that are close to the skin of the tree. It indicates recent development and has juice. It actively participates in the growth of the tree and the sap moves upwards through it.
Cambium: The thin layer of wood between the sapwood and the inner bark is called the cambium layer. If the bark is removed for any reason, the cambium layer is exposed and the tree dies.
Bark: This is the outermost region and forms the skin of the tree. The function of the bark is to protect the inner tissues from heat, rain and injury. Sometimes there is another thin membrane inside the bark and this is called the inner bark.
Medullary Rays: Thin radial fibers extending from the pith to the cambium are known as medullary rays. Its function is to hold the annual rings of hardwood and sapwood together.
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