In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting branches and leaves in most species.In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are usable as lumber or plants above a specified height.Wood gives structural strength to the trunk of most types of tree; this supports the plant as it grows larger.The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to grow as large as they do.Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil.Above ground, the branches divide into smaller branches and shoots.Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves.If insufficient water is available the leaves will die.
Trees tend to be long-lived, some reaching several thousand years old.
Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem.
The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss.
They differ from shrubs, which have a similar growth form, by usually growing larger and having a single main stem; made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas.
The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution.