Hyponasty

Hyponasty, the more rapid growth of the under surface of a dorsi-ventral organ in a plant, such as a leaf. It commonly alternates at long intervals with epinasty, the more rapid growth of the upper surface. In an ordinary leaf-bud, for example, hyponasty makes the young leaves at first arch inwards over the apex of the stem, and then epinasty causes them to spread outwards and downwards into a horizontally expanded condition. Horse-chestnut leaves are somewhat exceptional in epinasty in their case preceding hyponasty, so that the latter causes them to rise outwards and upwards into the horizontal.