| There are about 20,000 kinds of trees. More than 1,000 kinds grow in the United States. They range from mighty forest trees to fragile ornamentals. The greatest variety of trees grow in wet tropical regions.
Scientists who study plants divide plants with similar characteristics into various groups. These scientists, called botanists, do not put trees in a separate group of plants. Instead, each kind of tree is grouped with other plants that have certain features in common with it. Therefore, a group of plants may include certain trees, certain shrubs or vines, and certain herbs. For example, locust trees, broom plants, and clover all belong to the same family. These plants are grouped together because they reproduce in the same way and have similar flowers.
|
|
On the other hand, some trees that look much alike, such as tree ferns and palms, belong to different groups of plants.
Trees also can be divided into six groups according to various features they have in common. These six groups are: (1) broadleaf trees; (2) needleleaf trees; (3) palm, pandanus, and lily trees; (4) cycad trees; (5) tree ferns; and (6) ginkgo trees.
Broadleaf trees are the most numerous and varied of the world's trees. They include ashes, elms, maples, oaks, walnuts, willows, and many other familiar trees of the United States and Canada. They also include most trees of the tropics, such as mahogany trees and mangrove trees.
In addition to their broad, flat leaves, broadleaf trees have other features in common. Almost all broadleaf trees of temperate regions are deciduous-that is, they lose their leaves each autumn. A few kinds of broadleaf trees in temperate regions do not lose their leaves in the fall. These broadleaf evergreens include the holly trees and live oaks of the Southeastern United States. Some tropical broadleaf trees are deciduous, but most are evergreen.
Foresters call broadleaf trees hardwoods because many of these trees, such as beeches, maples, and oaks, have tough, hard wood. Such wood makes excellent furniture. Some broadleaf trees, including basswoods and cottonwoods, have soft, lightweight wood.
|
Broadleaf trees belong to a large group of plants called angiosperms. These plants have flowers which develop into fruits that completely surround the seeds. Fruits are the seed or seeds of a plant together with the parts in which they are enclosed.
Botanists divide angiosperms into two classes-Monocotyledonae (monocotyledons) and Dicotyledonae (dicotyledons). Monocotyledons produce seeds that have one leafy structure called a cotyledon. These plants include palm, pandanus, and lily trees. Dicotyledons produce seeds that have two cotyledons. These plants include broadleaf trees. A few kinds of trees that do not have broad, flat leaves belong to the dicotyledon group. An example is the saguaro cactus of the Southwestern United States, which has prickly spines.
Next >>> |
| Contributor: Richard H. Waring, Ph.D., Professor Emeritus of Forest Ecology, Oregon State University.
.. |
|
