Cambium
layer of actively dividing cells between xylem and phloem in plants
Cambium is a tissue in plants which is made of dividing cells. In trees it is found between phloem and xylem tissue.
There are several types of cambium depending on where it is in a tree or plant. Usually we only see the cork cambium, which makes the tough outer layer, the bark. Unlike the bark, cambium is made of living cells. They are one of the ways in which trees can go on growing for many years.