Tree (Encyclopedia of Science)
A tree is a woody perennial plant that has a single trunk arising from the ground (typically without branches near the base) and that usually grows to 20 feet (6 meters) or more in height. Branches and twigs grow from the trunk of a tree to form its characteristic leafy crown. Trees are the dominant plants in the world's forests, providing critical habitats for the other species that live there. Trees also provide many products that are important to humans, such as fruits, nuts, timber, and medicine.
Trees may be classified into two major groups: deciduous and evergreen (conifer). Deciduous trees shed their broad leaves at the end of the growing seasonypically each fall. Examples of deciduous trees are maples, oaks, and elms. Evergreen or coniferous trees typically have needle-shaped leaves that remain for several years before being replaced. Pines, firs, and spruces are examples of evergreens.
Structure of a tree trunk
Bark is the protective external covering of the stems (roots, trunk, and branches) of trees. The waterproof outer layer is known as cork. Composed of dead cells, cork can be as thick as several inches or more and serves to protect the internal living tissues from insects, animals, fungi, fire, and dehydration (the loss of water).
Underneath the cork is a layer of living tissue called the phloem...
(The entire section is 955 words.)
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