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. 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.