Answer:
In woody plants, some of the meristematic cells in the tips of the root and shoot stay meristematic. Usually, this ring of meristematic cells, the vascular cambium, is only one or two cell layers wide. Tissues produced by the vascular cambium are secondary tissues. . In older, woody parts of trees, the vascular cambium produces phloem to the outside, xylem to the inside, and more meristematic vascular cambium cells in between the phloem and xylem. The vascular cambium stays a very inconspicuous one or several layers in width but increases in ring size each year with the widening growth of xylem tissues. These increases, along with the bark, add to a tree's girth year after year.
