Tree

A tree is a tall, woody plant with one main trunk and many branches. It is perennial (grows for many years) and distinguished from a shrub by its size (usually 15 feet [4.6 meters] or taller) as well as by its single woody stem or trunk. A shrub is a short, woody plant that usually divides at its base. Trees are the most visible and dominate part of a forest. They also play an important role in the world's environment and economy.

Trees vary greatly in size, with the tallest being the redwood trees of coastal California that reach well over 350 feet (106.7 meters). Trees began their evolution more than 400,000,000 years ago. The oldest known living trees are the bristlecone pine (Pinus aristata) growing in the Rocky Mountains which are believed to be more than 4,500 years old. The widest tree is a giant sequoia (Sequoia gigantea) growing in central California. Nicknamed the "General Sherman," it has a circumference (measured completely around) of 115 feet (35.1 meters). Trees also make up the largest living organism in the world, with some stands or groups of trembling aspen trees covering as many as 100 acres (404,700 square meters). These trees reproduce by sending out roots that send up new sprouts that in turn become trees. Since the trees are all genetically identical and actually connected,

they can be said to represent a single organism. Trees are found in every region of the world except deserts, the Arctic, and Antarctica.

Woody plants, like trees, have tough stems covered by bark that do not die when the growing season has ended. It is their tough, woody stems that allow them to grow as tall as they do. Trees can be divided by the type of wood they form. Conifers (produced in cones) like pine and spruce are called softwoods, while most others are considered hardwood. A tree is like any other green plant in that it draws in water from its roots and makes food in its leaves through photosynthesis (the process by which plants use light energy to make food from simple chemicals). Deciduous trees, like maples and oaks, lose their leaves for a season and grow new, flat broadleaves each spring. Evergreen trees, like pines, are always covered with leaves (which have evolved into sharp needles), although they are constantly losing them and replacing them in small numbers. As with all plants, trees are either angiosperms (producing flowers and seeds with coverings), or gymnosperms (producing naked seeds). Conifers are gymnosperms since they produce seeds on modified leaf structures called scales that are on their cones. Most other trees produce flowers and are angiosperms. While spring flowers on a fruit tree are obvious and beautiful, the flowers on some trees do not resemble anything like a flower. The age of a tree can be determined by counting its rings after it has been cut down. Each year new wood is formed in a layer that is outside that of the previous year's wood growth. Very dry years result in thin rings, and years of good rainfall result in thicker rings.

Forests of trees have been called the "lungs of the world," since they provide an enormous amount of oxygen to the environment as a by-product of photosynthesis. Besides providing oxygen, trees and the forests they make up form the habitat for many animals, and virtually every part of a tree has been put to some use by humans—even a tree's sap and bark. Trees supply wood for fuel as well as for lumber, paper, and plastic products. Humans dependence on trees for products has begun to have a negative impact on forests. Today, deforestation is a growing problem, especially in less developed countries where trees are being cut down faster than they are able to grow.

[See alsoForest; Plants; Rain Forests ]