ecause trees have such varied features, there are numerous ways to classify them. They can be grouped according to the way they shed their leaves, how they reproduce, how they grow, their internal structure, and the types of flowers and seeds they have.

The broadest way to categorize trees is by their seed development. Gymnosperms (JIM-no-sperms) are plants that have "naked" seeds -- seeds that are not enclosed in flowers or in fruit. In most gymnosperms, the seeds are produced on the surface of the scales of female cones and are pollinated by wind. Conifers -- such as pines, hemlocks, redwoods, spruces and firs -- are the most common types of gymnosperms with more than 500 species worldwide.

Angiosperms (AN-gee-oh-sperms): Worldwide there are more than 235,000 species of angiosperms, the only types of plants that have true flowers and bear their seeds in fruits. In temperate zones, many angiosperms -- such as oaks, willows, maples and birches -- are deciduous trees. In tropical zones, many angiosperms are evergreens. These include palms, figs and southern magnolia. All flowering plants that are not trees, such as tulips, blackberries and poppies, are also in this family.

Foresters often call broad-leaved trees "hardwoods" because most broad-leaved trees have harder wood than do most needle-leaved trees. For example, maples and oaks are known for their tough, hard wood and are often used to make high-quality furniture and floors. Of course there are always exceptions such as cottonwoods and magnolias, which are broad-leaved but have very soft, lightweight wood. Softwood is used to describe needle-leaved trees such as pines and spruces because most of them have softer wood than do most broad-leaved trees. An exception is yellow pine, which is a needle-leaved tree but has very hard wood.