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Abscission

    Abscission (from Latin abscindere, from ab- ‘off, away’ + scindere ‘to cut’) 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 may also be variously involved in the process. Auxin is a plant hormone which can prevent the formation of abscission layers and premature fruit drop. Auxin is also believed to play apart in the shedding of leaves and there color chage in the fall from woody plants, when the adscission zone or layer cuts off the movement of auxin from the leaf blade to the leaf.
    In deciduous trees, an abscission zone, also called a separation 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 weak walls of the cells in the top layer. This allows the leaf to be shed.
    In woody plants a abscission layer is formed composed of parenchyma cells bounded on both side with cork. this layer is found at the base of the leaf petioles in woody angiosperms and gymnosperms and do to the disintegration of the parenchyma layer the organ such as a leaf or bark is separated from the parent plant. Abscission is a natural process of plant growth induced by the plant, in contrast to decaying or falling off do to other causes.
    The liberation of a fungal spore by the withering away of an adjoining layer is also called abscission.