Bark often acts as the outer sheath of many types of trees and shrubs. It continues to protect the roots, stems, and branches, from loss of water, disease, insects, and injury. Tissues in the bark transfer sugar from the trees leaves to other areas of the plant.
Bark is composed up of circular layers of tissues that surround the outside of the wood core of the tree or shrub in question. These tissues are sectioned into two parts, inner bark, and the outer bark. Tissues of the inner bark carry and store the trees food. The other bark works to act as a protective coating for the plant.
This is comprised of several living, and developing tissues. These layers in order from inner most to outer most are (1) phloem, (2) phelloderm, and (3) cork cambium or otherwise known as phellogen. The phloem is made up of sieve tubes, which extract the sugar from the leaves down to the rest of the tree. These banded fibres may aid these tubes. The phloem also contains a variable of other cells, including the ray and companion cells. The phloem found in older wood plants is produced by the tissue known as the cambium. The cambium can be found between the bark and wood. The cell division in the cambium produces new layers of inner bark and wood. This cell division has been known to cause the stems of other plants to grow much wider. The phelloderm is the layer of food keeping cells. This is produced by the cork cambium, which works similar to the cambium in the manufacturing of new tissues. The development of new phloem expands the phelloderm and the cork cambium, until it breaks apart, and withers away. To replace the dead tissues, new layers of phelloderm and cork cambium begin to develop in place.
The outer bark is largely consisted of cork, which is a tough, dead tissue produced by the cork cambium. Segments of dead phloem happen throughout the outer bank of older trees and shrubs. This dead phloem is then pushed out by the development of new phloem. The cork cells present have extremely thick walls, which consist of a waterproof, waxy, substance called suberin. This suberin works to aid the plan from losing water and the capturing of necessary gases, to prevent them from coming in and out. These gases tend to leave and enter the stem through lenticels, which are oval blisters in the barks surface. In much more adolescent stems, the outer bark is marked by scars at points where leaves and buds had once attached themselves to. The cork cambium makes new layers of cork almost annually.
Keating, Richard C. (1995) Bark. World Book,