When you cut a tree trunk, you'll see several concentric rings. Each ring and layer is an integral part of the tree. Both softwood trees and hardwood trees have a trunk composed of five different sections. Each element serves its own purpose, protecting the tree and promoting further growth throughout the year.
Bark is the outermost layer of a tree trunk. Bark protects the tree trunk from the outside world, preventing damage from cold and sun in all trees, although the quality of bark differs among trees. The outer bark keeps moisture within the tree and keeps out insects.
The phloem is the inner bark of the tree. Phloem acts as a transport for food produced in the leaves, passing it to the living parts of the tree. The phloem is a series of living cells that have a short life span, says says the University of Kentucky. Once phloem dies off, it becomes part of the outer bark.
Between the bark of the tree and the inner wood is a layer of cells containing protoplasm called the cambium. The cambium is a growing part of the trunk. The layer itself is one cell thick, and produces bark on one side and wood on the other. Leaves produce hormones called auxins. Auxins are transported from the leaves to the cambium by the phloem. The auxin hormone promotes the growth of new bark and wood.
Sapwood is the new wood produced by the cambium. New wood works as a food transport system, moving water and nutrients from the soil into the leaves where photosynthesis occurs. Sapwood lives for one year before being covered by a new layer of sapwood, notes North Carolina State University.
Sapwood loses its vitality and hardens when it is covered by a new layer of sapwood. This forms heartwood, which is in the center of the tree. Its dead cells do not decay, making a strong foundation for the rest of the tree to grow around.