Forest Preserve District of Cook County, Illinois
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Bark
Nature Bulletin No. 594   March 5, 1960
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist

BARK
A tree has three main parts: the roots, the trunk with its branches, and the leaves. The trunk has a dense core of heartwood that gives it strength. Surrounding that is a layer of sapwood. On the outside is a layer of bark. The bark protects the wood against attack by fungi and, most important, it protects the cambium: a microscopically thin layer of wonder-working cells between the sapwood and the bark. If a tree is "girdled" -- its wood exposed by removing a band of bark around the trunk--the cambium dries and the tree dies.

The cambium enables a tree to live and grow. Each year it builds another layer of sapwood and also a layer of bark. The sapwood conducts water and dissolved nutrients, taken from the soil by the roots, up through the trunk and branches to the leaves. Food manufactured by the leaves is carried down to the trunk and roots through the spongy sieve-like inner bark. As new layers of wood and bark are added, the older growths of bark are pushed outward. They become dry and hard. Eventually they become loose and drop off.

In each species the bark on the older trees has a distinctive appearance Even during winter, many of our hardwoods can be identified by the color of the bark and whether it is smooth, ridged, deeply furrowed, scaly, or shaggy. The beech, the paper or canoe birch, both hornbeams, the sycamore, hackberry, white oak, bur oak, black cherry, and shagbark hickory are familiar examples. On some kinds the bark is rather thin; on others, such as a bur oak, it may be 2 inches thick; on a gigantic sequoia the deeply wrinkled bark, spongy and fire resistant, frequently has a total thickness of 2 feet.

Our American Indians had some use for one or more parts of almost every plant, including the bark of trees and shrubs. Sheets of bark peeled from the paper birch were used to cover Ojibwe wigwams and canoes. It was used to make all sorts of baskets, buckets, trays and vessels for gathering, cooking and storing foods. The Potawatomi commonly used elm bark to make utensils and cover their wigwams. Both tribes used fibers from the inner bark or bast of the linden (basswood) for cordage and weaving. For various medicines they used bark from hemlock, tamarack, pine, spruce, black and choke cherries, poplars, willows, slippery elm, speckled alder and buckthorn They made dyes from the bark of sumac, speckled alder, birch, oak, hemlock, willows and wild plum.

Marco Polo told how, in the 13th century, the Mongols made paper money out of the inner bark of the paper mulberry, now used in Japan for making paper and in the South Sea islands for tapa cloth. Cinnamon, the aromatic inner bark of evergreen trees native in Ceylon and India, has been prized for centuries as a spice and as an ingredient in perfumes and incense. For tanning leather we have used, since colonial times, the astringent barks of chestnut, oaks and hemlock' now largely supplanted by quebracho bark from Argentine and Paraguay. The bark of cinchona trees, native in Peru, furnishes quinine.

Cork. an extremely light, buoyant substance which has many important uses, is obtained in Portugal from the outer bark of the Mediterranean cork oak. During World War II, the bark of Douglas fir was found to be valuable for many purposes and is no longer wasted. The thick shaggy bark of our California redwoods is now utilized as material for insulation, floor cleaners, and as a substitute for wool in fabrics.

The bark of a tree has been transposed into clinks of the almighty dollar.


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