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Abscission (from ab- away from, and scission cutting or severing) is the shedding of a body part. It most commonly refers to the process by which a plant intentionally drops one or more of its parts, such as a leaf, fruit, flower or seed, though the term is also used to describe the shedding of a claw by an animal. A plant will abscise a part either to discard a member that is no longer necessary, such as a leaf during autumn, or a flower following fertilisation, or for the purposes of reproduction. If a leaf is damaged a plant may also abscise it to conserve water or photosynthetic efficiency, depending on the 'costs' to the plant as a whole. The gaseous plant hormones ethylene can stimulate abscission. Abscisic acid and auxins may also be variously involved in the process. in deciduous trees, an abscission zone, also called a seperation zone is formed at the base of the petiole. It is composed of a top layer which has cells with weak walls, and a bottom layer which expands in the autumn, breaking the weaks walls of the cells in the top layer. This allows the leaf to be shed.
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