The Skin (or Bark) of a Tree

Bark is a word used to define a tree's outermost covering. It is typical to think of bark as having a fairly simple structure. However, this relatively complex covering is composed of an outer layer of dead cork cells, and a second layer of living cork cambium and phloem cells. The outer layer of cork has a characteristic structure for each species, but in all species of plants this layer is relatively impervious to water, and therefore helps to prevent loss of moisture through the bark. In healthy specimens, the cork is also somewhat resistant to penetration by insects, fungi, bacteria or viruses, and therefore is an important part of the plant's defense against disease. The cork cambium produces new cork cells, often from ruptured phloem cells that have been replaced by new tissue, and is responsible for keeping the tree's outermost covering intact.

The phloem, which forms the inner bark, or bast tissue, of the tree, is even more complex than that of the outer bark. It is vascular, that is, it consists of tubes or ducts that carry fluids. The phloem tissue is the primary conductor of food nutrients throughout the plant. Xylem tissue, which is the innermost layer of tissue in the cambium, is also vascular, is composed of stiffer materials, and has a fibrous structure that gives it strength. Xylem conducts water and certain nutrients throughout the plant, although mostly from the roots upward. As xylem tissues die, they are replaced with new tissue by the inner cambium. For a time, the dead xylem tissues continue to perform their earlier vascular duties, and during this period they form, collectively,  what is commonly known as sapwood. Eventually, the oldest xylem tissues become clogged with wastes, and cease to transport fluids. This tissue forms the central portion of the tree's stem, and is called heartwood.

Healthy cambium constantly produces new layers of phloem on the outside and xylem on the inside, causing the plant stem to grow larger. 

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