John Charles Milazzo, age 9, of Marquette, Mich., for his question:
WHY DOES A TREE HAVE BARK?
Bark is the outer covering of the stem of trees and other woody plants. One of the main functions of the bark of trees and shrubs is the protection it gives the inner, more delicate structures. Bark not only keeps them from drying out, but also guards against outside injuries of various sorts.
Often it is difficult to decide how much of some stems should be referred to as bark. In broadleaved plants, such as the common maple tree, it is easy to tell the wood from the outer and darker bark. But in other plants, such as the palm tree, it is more difficult to know how much of the hard and corky outer portion to call bark. In the palm, there is no clear line of separation between bark and wood.
In the young branch of a maple tree there is at first no rough bark. The surface of the shoot is nearly smooth.
However, as the twig forms more wood and grows in size, the outer portions may be split open. The injury caused in this way is healed from the inside. Some of the outer portions become dry and die. The dead broken portions give the bark a rough appearance.
As the tree grows, some of the dry pieces are shed or broken off. The process by which bark is formed may go on year after year.
Some bark has commercial value. Commercial cork, for example, is obtained almost exclusively from the cork oak tree. This tree grows in Spain and other regions close to the Mediterranean.
The bark of the hemlock tree is used in the tanning of leather. The spice called cinnamon comes from the powdered bark of a tree which grows in India and Malaya. And quinine is obtained from the bark of the cinchona tree.
The bark of many other kinds of trees and herbaceous plants are used as medicines.
Some of the large California redwood trees are at least 3,000 years old. The bark on these trees has continued to develop through all that time. The bark of such aged trees is very thick in spite of the fact that much of the old, outer portion has been worn away.
The thick fibrous bark of many of the old redwood trees shows burns and scars as a result of fires near the ground, but the inner portion of the trees escaped injury.
Frequently it is possible to tell the family or even the variety of a tree by the character and appearance of its bark. For example, it is very easy to distinguish between a maple, a beech or a pine tree by examination of the bark alone.
Extracts from the bark of some trees are used for flavoring. A good example is found in the sassafras which is used to make a mild aromatic tea. An extract of the sassafras is also added to some soft drinks such as root beer.
The canoe birch furnished the bark used by the Indians for making canoes.
The inner fibers of the bark of the basswood tree were once used very commonly for making twine. They were also woven into coarse cloth.
Many animals, such as the beaver and the rabbit, live in part on the bark of trees and various shrubs.