Cambium (plural cambia) is the botanical term for the delicate tissue within a tree between the inner wood and bark.

It is responsible for the secondary growth of a tree, its radial growth or width, seen in cross-section as tree rings.  

Over time, the development of cambia provides strength and stability to a tree so that it may support itself to grow tall, be resilient, and resist the stresses of nature such as wind and storms. As the cambia grows over the lifespan of the tree, it also allows the tree to better nourish itself and ensure its longevity. 

Through cambial growth the tree yields to the inevitable process of expansion and renewal; new wood and bark are formed over time. The old bark that is no longer useful expands, splits and falls away, revealing new skin underneath, created by the actions of the cambium.