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A tree is a plant consisting of roots, one or more large stems called trunks, branches, twigs and leafs. In analogy with skin, a tree has bark to protect it against infections and fungus attacks, and also to transport nutrients and water to the growth zones along the branches and leaves. A small group of trees growing together is called a grove whereas a large population of trees is called a forest.

The roots of a tree stand embedded in earth and extract water and nutrients from the surrounding soil. Above ground, the trunk gives height to the leaf-bearing branches in order to compete for sunlight. In many species of trees, the branches spread to present the largest possible leaf surface to the sun.

A tree may not have all of the organs mentioned here: most palms do not have branches, the saguaro of North America has no functional leaves, and tree ferns don't have bark. Based on their rough shape and size, all these are considered trees nonetheless.

Some trees can grow to over 100 meters of height and/or become several millennia old if circumstances are optimal. A smaller variant of tree, generally with smaller, or multiple trunks, is often called a shrub, though there's no precise boundary between the two categories.

Several types of biotopes are defined largely by the trees that inhabit them: The rainforest and the taiga are examples.

Trees often serve as important symbols in mythologies and religions. Examples are Yggdrasil in the Norse Mythology and the Tree of Knowledge of christianity.

Commonly known variants of trees are:

In computer science, the term tree denotes a type of graph which consists of a single "root" node to which a branching pattern of child nodes are attached. See Tree data structure.