More rapid growth of the lower side of an organ than of the upper side. 
• (n.) Downward convexity, or convexity of the inferior surface.
More rapid growth of the lower side of an organ (e.g. a petiole) than of the upper side.
Hy`po·nas'ty noun (Botany) Downward convexity, or convexity of the inferior surface. 
hyponasty 1. The state of growth in a flattened structure in which the under surface grows more vigorously than the upper side. 2. An upward bending of leaves or other plant parts, resulting from growth of the lower side. In plant physiology, the state in which more vigorous growth occurs in the lower surface of an organ, such as a young fern fro...
increased growth along the lower surface of a plant or plant part, causing it to bend upward.