| Tree is the largest of all plants. The tallest trees grow higher than 30-story buildings. Many trees also live longer than other plants. Some trees live for thousands of years. They are the oldest known living things.
People do not think of trees the way they think of other plants, most of which grow only a short time and then die. People think of trees as permanent parts of the landscape. Year after year, large, old trees shade houses and streets from the sun. Their buds and flowers are a sign of spring each year, and their colorful leaves brighten in autumn in many areas.
Trees continue to grow as long as they live. A tree's leaves make food that keeps the tree alive and helps it grow.
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Where winters are cold, many trees lose their leaves in autumn. Other trees keep their leaves during the winter and so stay green all year long. Trees that shed their leaves in autumn rest during the winter. In spring, they grow new leaves and flowers.
The flowers grow into fruits, which contain seeds for making new trees. Some tree fruits, such as apples and oranges, taste good. Fruit growers raise large amounts of these fruits for sale. Trees also make new wood each year when the weather turns warmer. Wood is one of the most valuable parts of a tree. Mills and factories use wood to manufacture lumber, paper, and many other products.
A tree differs from other plants in four main ways. (1) Most trees grow at least 15 to 20 feet (4.6 to 6.1 meters) tall. (2) They have one woody stem, which is called a trunk. (3) The stem grows at least 3 to 4 inches (8 to 10 centimeters) thick. (4) A tree's stem can stand by itself. All other plants differ from trees in at least one of these ways.
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For example, no plant with a soft, juicy stem is a tree. Most of these plants, called herbs, are much shorter than most trees. Shrubs, like trees, have woody stems. But most shrubs have more than one stem, and none of the stems grows so thick or so tall as a tree trunk. Some jungle vines grow more than 200 feet (61 meters) long and have a woody stem. But the stems of most vines cannot support themselves.
There are thousands of kinds of trees. But most trees belong to one of two main groups-the broadleaf trees and the needleleaf trees. These two types of trees grow in Europe, North America, and many other parts of the world. Most other types of trees, such as palms and tree ferns, grow mainly in warm regions.
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Contributor: Richard H. Waring, Ph.D., Professor Emeritus of Forest Ecology, Oregon State University.
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