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more rapid growth of the lower side of leaves, petals, sepals, and other organs of plants in comparison with the upper side. As a result of hyponasty, a given plant organ bends upward. Hyponasty may be conditioned by the age of the organ. For example, young flower buds remain tightly closed because of hyponasty. This phenomenon is superseded by epinasty—that is, more rapid growth of the upper side of the sepals and petals, as a result of which the flower opens. Hyponasty may also be caused by changes in environmental conditions. For example, during overcast, wet, or cold weather hyponasty of the perianths sometimes occurs, and the flowers close.