Cambium - the growing layer of a tree made up of microscopic cells. The cells divide, producing phloem to the outside and xylem to the inside. Xylem - transports sap upwards from the roots to the leaves. Also known as sapwood. This forms when the xylem cells plug up with resins and minerals.
Heartwood gives support for the plant.
Green plants are the only things in nature that can actually make their own food. Photosynthesis is the chemical process that takes place in the leaves acting as food factories. This chemical process needs carbon dioxide, water, minerals, chlorophyll, and sunlight, and produces sugar. Additional by-products are oxygen and water vapor through transpiration. Bark - protection from insects, disease, fungus, and other natural forces.