During this time of the year the bark of a tree is very loose and can be easily knocked off the tree. Inside the vascular cambium is the largest portion of the trunk known as the xylem. The xylem is composed of dead, thick cells that act as pipes for transporting water and nutrients up the tree. The active portion of the xylem is known as the sapwood. The sapwood is found near the outside of the tree.
Older xylem is known as heartwood and is found in the middle of the trunk.
The heartwood is often darker in color and is not very active in transport. The darker color is caused by resins, oils and minerals, which are deposited as the tree grows.