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Tree, perennial plant having an upright woody main stem, and usually the tallest of plants at maturity. A tree differs from a shrub in that it usually produces a single main stem, or trunk, and from an herb in that the stem is composed almost entirely of woody tissue. Trees of some smaller species sometimes develop with more than one stem, like a shrub, but most species of larger size grow only in tree form. Some species, when they reach maturity, are only 4.6 m (15 ft) high, with trunks as slender as 15 cm (6 in) in circumference; the largest species may reach higher than 112 m (367 ft), with trunks that have a diameter of more than 6 m (more than 20 ft). Trees are popularly grouped into two broad categories: evergreen and deciduous trees. (These categories do not correspond strictly to the scientific classifications described below.) Evergreens are those that bear foliage throughout each year, constantly shedding a small proportion of the older leaves and replacing them with new leaves. Two evergreen leaf types are common: (1) needle leaf, typified by the tough, narrow or scalelike resinous leaves of most conifers; and (2) broadleaf angiosperms, most common in tropical areas, but found in temperate areas as well. Deciduous trees are broad-leaved and lose their foliage each year, usually at the approach of the coldest and/or darkest season.

All trees are seed-bearing plants, either gymnosperms, mostly cone-bearing plants commonly called softwoods, or angiosperms, which are flowering plants, the trees of which are commonly called hardwoods. The angiosperms are further divided into two classes, the Liliopsida (monocots) and the Magnoliopsida (dicots), depending on seed structure. Of the 60,000 to 70,000 species of trees, all are dicotyledonous except a few hundred monocotyledonous species and less than 1000 gymnospermous species. All five living orders of gymnosperms consist primarily of tree species; the most important gymnosperm orders are the Pinales and Taxales, comprising the conifers. Among the angiosperms, few tree species are monocots. The only monocotyledonous family containing a preponderance of tree species is the palm family, Areceae, the genera of which are native to tropical and subtropical regions throughout the world. The dicots include most of the broad-leaved trees, which are distributed throughout the world. In the U.S., native trees belong to about 850 species, which are classified in 222 genera and 69 plant families. Of this total, about 300 species belong to the oak genus, Quercus. Other large genera of American trees are Crataegus, the hawthorns; Pinus, the pines; and Salix, the willows. About 110 other species native to the U.S. are tropical or subtropical trees restricted to Florida. In addition, about 60 species of trees from Europe and Asia are naturalized in the U.S. after centuries of cultivation, and more than 200 species of foreign trees are commonly grown in the U.S. as ornamental, shade, and fruit trees. Tree species make up about 3.5 percent of the plant species found in the U.S.

Trees have existed since the Devonian period of the Paleozoic era . The oldest trees known to paleobotanists are those of the genus Cordaites, which originated in the early Devonian period and became extinct by the end of the Paleozoic era. The oldest known surviving order of trees, the broad-leaved, gymnospermous Ginkgoales, is now represented by a single species, the maidenhair tree, Ginkgo biloba . Coniferous trees have existed since the middle of the Carboniferous period. Angiospermous trees first appeared in the lower Cretaceous period of the Mesozoic era, and by the beginning of the Pliocene epoch of the Cenozoic era virtually all tree genera now in existence were growing profusely. The majority of fossil tree leaves found in Pliocene rocks are indistinguishable from leaves of present-day trees.

Climate and Soil Requirements
Trees grow wherever adequate groundwater is available for the major portion of the year. Trees do not grow profusely in desert areas or in areas in which the groundwater table is sufficient only for grassland vegetation; in such areas trees grow successfully only under careful cultivation, in desert oases, or along the banks of rivers and streams. Moreover, in areas bordering a grassland or desert, trees are frequently stunted and gnarled in growth. In high mountains or at the edge of the northern coniferous forests, such scattered, stunted, twisted trees are called krummholz. Under optimum conditions, however, trees grow in large aggregations called forest. The climatic and soil requirements of trees are somewhat different for each species. Most tree species grow over large areas, of which only a small proportion permits optimum growth of the plant. The most common tree species in a given area is called the dominant species of the area. In the eastern U.S., for example, spruce and fir are dominant in Maine and northern New York; beech, birch, and maple in the southern portions of New York, Michigan, and Wisconsin; longleaf, loblolly, and slash pines in the Gulf states; and oak and hickory in most of the remaining areas of the eastern U.S. Some of the states have chosen the blossoms of characteristic native and cultivated trees as state flowers; apple, Malus pumila, for example, is the state flower of both Michigan and Arkansas.

Basic Structure
Growth of trees, like that of shrubs, requires the successive addition of many layers of woody tissue to the stem of the original young seedling. The axis, or root and stem, of a tree seedling is divided into three main layers. The outermost layer, called the epidermis, is composed of thin-walled cells and protects the inner layers of the axis. The middle layer, called the cortex, is composed of larger, thin-walled cells, which function temporarily as storage cells. The innermost layer, or stele, is composed of a layer of tough pericyclic cells, a multicellular layer of phloem cells, a multicellular layer of xylem, or wood, cells, and an inner core of thin-walled cells that is called pith. Embryonic Growth Early in the development of the plant, an embryonic layer of cells, called the cambium, develops between the phloem and xylem layers. The cambium layer alternately produces additional phloem and xylem cells by constant division. When a cambium cell divides to form xylem cells, the inner of the two resulting cells develops into a xylem cell. The outer cell continues to function as cambium in the next division, in which the outer cell develops into a phloem cell and the inner cell continues to function as cambium. Many more xylem cells are produced than phloem cells.

The constant divisions of the cambium gradually increase the circumference of the axis. The cambium continually increases in circumference as the area of wood enlarges from the increase in xylem cells, but the tissues outside the cambium—namely, phloem, pericycle, cortex, and epidermis—soon rupture, form deep fissures, and eventually split off. A new cambium, called the cork cambium, or phellogen, develops outside the phloem and produces successive layers of cork cells that protect the axis. As the axis continues its expansion, the layers of cork frequently develop characteristic fissures at the surface, and as each cork cambium is split by the expansion of wood, a new cork cambium develops to replace it.

At maturity, the tree axis normally consists of several layers of cork cells, the outer portion of which is fissured: the cork cambium, a few layers of crushed phloem, a few layers of functioning phloem, the cambium, and many layers of xylem. The xylem layers usually constitute more than 95 percent of the diameter of the axis. The xylem layers are collectively called wood, and the layers outside the cambium are collectively called bark. The cork cambium divides the bark into outer bark and inner bark.

Annual Rings
Because the xylem cells produced in the spring of the year are large and those produced later in the year are smaller, and because an interruption of growth occurs during the winter, the growth of wood for each year appears as a distinct ring, called an annual, or growth, ring. The width of each ring is affected by climate and other variables, and archaeologists have studied tree rings to determine the climatic conditions and variations in environment of former times. By starting with trees the ages of which are known and comparing their rings with those of trees of unknown age, archaeologists have worked out a chronology extending back some 4000 years. This tree-dating method, called dendrochronology, has been used to date ancient structures and buildings the wooden beams of which have been preserved. Older annual rings are usually darkened and nonfunctional, and are collectively called heartwood; younger layers are lighter in color and function in transporting sap, and are therefore called sapwood.

The sapwood of the axis functions to carry water and dissolved mineral nutrients upward from the soil to the leaves. In the leaves the water is used, in combination with carbon dioxide taken in through the leaves, in a process of food manufacture called photosynthesis. The sapwood also transports the gaseous products of respiration, which occurs in all living cells of the plant, to the leaves, from which the gases are released into the atmosphere. Food, manufactured by photosynthesis, and oxygen, absorbed from the air and used in respiration, are transported downward to the roots by the phloem.

Reproduction in trees, as in almost all other plants, involves the alternation of generations. Ovules and pollen may be borne in the same flower or the same inflorescence of a single tree. In most hollies, ashes, and maples, and in yew, juniper, and ginkgo, however, the trees are either “male” or “female.” Trees are usually wind- or insect-pollinated, but several species of birch produce fertile seeds without pollination.

Life Span
The normal age span of trees is different for each species. Some birches, for example, normally die after about 40 years of life. The sugar maple, on the other hand, frequently lives as long as 500 years, some oaks as long as 1500 years, some junipers as long as 2000 years, and some giant sequoias as long as 4000 years. Bristlecone pines, some almost 5000 years old, are the oldest living things.


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