The trunks of trees consist of different layers. When looking at a cross-section image of a pine trunk, for example, we can notice distinct layers that run through the length of the trunk.

The outermost layer of the tree trunk consists of bark. Its function is to provide protection for the more vulnerable and vital parts of the tree's conducting or vascular tissue. These consist of phloem tissue,  the function of which is to transport sugar between different parts of the plant, and the innermost xylem tissue, the function of which is to transport water and nutrients between different parts of the plant. A layer of cambium separates the phloem and the xylem. It consists of thin-walled cambial cells that can divide and differentiate into either xylem or phloem cells. 

The cambial cells divide and differentiate the most during the spring. During this time, the innermost part of the tree develops new layers of light, young xylem tissue. During the summer, the divison of cambial cells slows down, resulting in a darker layer of xylem growth. Together, these light and dark layers of xylem form the tree's growth rings, which can be used to determine the age of a tree.