The outer layer is the skin or bark. It minimizes moisture loss and protects the inner layers. The second layer is the cambium. It is the growing layer of the plant and is vitally important. Although only a fraction of an inch thick in even the largest trees, the cambium provides all the new cells in a plant.
Cells produced on the outside of the cambium layer become bark.
Cells produced on the inside of the cambium layer become vascular bundles and transport food and water to the leaves. The vascular bundles are supported in the third or inner layer by pith cells that die and become lignified. The lignified cells become the heartwood of the plant. Diseased or damage plant tissue cannot heal, Instead the plant forms a natural barrier to disease between the diseased area and healthy tissues, isolating the main part of plant. The barrier is formed by a chemical action and is reinforced over time.