Bark is a protective, outer tissue that occurs on older stems and roots of woody coniferous and angiosperm plants. Bark is generally considered to occur on the outside of the tissue known as wood, or the water-conducting xylem tissues of woody plants. The inner cells of bark, known as phloem, grow by the division of outer cells in a generative layer called the vascular cambium, located between the bark and wood (inner cells of this cambium produce xylem cells). The outer cells of bark, known as cork, grow through cellular division in the cork cambium, present outside of the phloem. The outer part of the bark is a layer of dead cells, which can be as thick as several inches or more, and serves to protect the internal living tissues from injury, heat, and desiccation.
The macroscopic structure of bark varies greatly among species of woody plants. For example, the bark of American beech (Fagus grandifolia) is distinctively grey and smooth. In contrast, many species have a deeply fissured, rough bark, as in the cases of sugar maple (Acer saccharum) and spruces (Picea spp.). The color and pattern of fissuring and scaling of bark can often be used to identify species of trees and shrubs.
Some types of bark have specific uses to humans. The young, brownish, inner bark of young shoots of the cinnamon tree (Cinnamomum zeylanicum), native to Sri Lanka but now widely cultivated in the humid tropics, is collected, dried, and used whole or powdered as an aromatic flavoring of drinks and stews. Some barks have medicinal properties, such as that of the cinchona tree (Cinchona calisaya), from which quinine has long been extracted and used to reduce the fevers associated with malaria. More recently, taxol, an anti-cancer chemical, has been identified in the bark of the Pacific yew (Taxus brevifolia). Taxol is now used to treat ovarian cancer, and is also effective against some other cancers.
The bark of some species of trees contains large concentrations of a group of organic chemicals known as tannins, which can be reacted with animal skins to create a tough, flexible, and very useful material known as leather. Tannins will also react with certain metal salts to form dark pigments, which are used in printing and dyeing. Major sources of tannins in North America are the barks of hemlock trees (especially Tsuga canadensis and T. heterophylla), and oaks, especially chestnut oak (Quercus prinus) and tanbark oak (Lithocarpus densiflora). Eurasian oaks and hemlocks are also used, as are several tropical species, such as red mangrove (e.g., Rhizophora mangle) and wattle (e.g., Acacia decurrens). The thick, outer bark of the cork oak (Quercus suber) of Europe is collected and used to manufacture bottle corks, flotation devices, insulation, and composite materials such as parquet flooring. The bark of some conifers is used as a mulch in landscaping, for example, that of Douglas fir (Pseudotsuga menziesii) and redwood (Sequoia sempervirens).