Trunk Trees Explained
In order to properly prune a tree or to repair damage to the trunk requires a basic understanding of tree structure. A cross section of a tree trunk reveals it is composed of many layers (Figure 1). Each year a tree essentially grows a new “coat of wood” over the older wood. The outside layer of the tree is dead bark which provides protection from the environment. The inner bark layer is composed of live tissue that transports food downward. Between the bark and wood is the cambium layer which is responsible for increases in tree diameter (by creating annual rings) and responds to injury by producing callus tissue. 
The annual rings of wood are composed of large pores that carry water up to the leaves. Each annual ring is essentially a vertical cylinder. The outer 4 to 20 annual rings (referred to as sapwood) are usually alive and light-colored. Wood in the center of a large tree (referred to as heartwood) is composed of dark-colored, dead cells used for storage. Ray cells cut across the annual rings; they distribute food to living cells. 
Branches
Branches are attached to the tree trunk by interlocking branch and trunk tissue. A new layer of interlocking tissue is produced each year over the previous layers. A woody branch collar, produced by the trunk, holds the branch base. When branches on the main trunk that have a narrow angle increase in diameter they eventually run out of room to grow. The branch bark becomes surrounded by woody trunk and branch tissue. The bark that becomes overgrown is referred to as included bark (Figure 2). The union is weak and likely to split.