tree, perennial woody plant with a single main stem (the trunk, or bole) from which branches and twigs extend to form a characteristic crown of foliage. In general, a tree differs from a shrub in that it has a single trunk, it reaches a greater height at maturity, it branches at a greater distance from the ground, and it increases in size by producing new branches and expanding in girth while a shrub often produces new shoots from ground level. Trees fall into three major divisions: angiosperms, gymnosperms, and pteridophytes. Angiosperms are the most common type, where seeds carried in various fruits are the agents of reproduction. Trees and shrubs may be deciduous, with broad leaves that are shed at the end of the growing season, or evergreen (see conifer), with needlelike or scalelike leaves that are shed at intervals of between 2 and 10 years, thus maintaining green foliage at all seasons. Trees are identified both by the characteristic color and shape of the leaf and by their overall appearance, e.g., the degree and angle of branching, the shape of the crown, and the texture of the bark. Their age can be determined from a count of the annual rings, which represent the diameter growth of a tree each year. Besides their enormous importance in providing oxygen and moisture for the atmosphere and removing harmful carbon dioxide, trees are an important source of food, of wood, and of numerous products (e.g., resins, rubber, quinine, turpentine, and cellulose for the manufacture of paper and various synthetic materials) derived from their wood, bark, leaves, and fruits.
See H. Johnson, The International Book of Trees (1973) and The World of Trees (2010); L. Line and A. Sutton, Audubon Society Book of Trees (1981); A. C. Barefoot and F. W. Hankins, Identification of Modern Tertiary Woods (1982).
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