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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. In wider definitions, the taller palms, tree ferns, bananas, and bamboos are also trees. Trees are not a taxonomic group but include a variety of plant species that have independently evolved a trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be long-lived, some reaching several thousand years old. Trees have been in existence for 370 million years. It is estimated that there are just over 3 trillion mature trees in the world. A pergola is an outdoor garden feature forming a shaded walkway, passageway, or sitting area of vertical posts or pillars that usually support cross-beams and a sturdy open lattice, often upon which woody vines are trained.[1] The origin of the word is the Late Latin pergula, referring to a projecting eave. As a type of gazebo, it may also be an extension of a building or serve as protection for an open terrace or a link between pavilions. They are different from green tunnels, with a green tunnel being a type of road under a canopy of trees. Pergolas are sometimes confused with arbours ("arbors" in American English), and the terms are often used interchangeably. An arbour is generally regarded as a wooden bench seat with a roof, usually enclosed by lattice panels forming a framework for climbing plants. A pergola, on the other hand, is a much larger and more open structure and does not normally include integral seating