The part of a tree trunk that we can see is the outer bark. The outer bark protects the tree from extreme temperatures, bad weather, insects and fungi. The phloem also called the bast is the inner bark. It conveys the food bearing sap developed in the leaves down to the various parts of the tree.
The major living portion of a growing tree is a thin film of cells in its trunk called cambium.
Sapwood is the living wood in the tree through which the raw sap rises from the roots to the leaves. The heartwood consists of old cells. This is the dead part of the trees that nevertheless provides structural strength.
The tree forms new cells, arranged in concentric circles called annual rings or annual growth rings. In each year that the girth of a tree increases, a new ring grows just under the bark. So, the age of a tree can be calculated from the number of rings under the bark.