Tropism << TROH pihz uhm >> is a bending movement in living things caused by an outside stimulus (signal). For example, sunlight causes plant leaves and stems to bend toward it. A tropism results from differences in the growth rate of various parts of an organ (a body structure) when the stimulus is present. The bending usually is slow. For example, some tropisms in plants take from one hour to several days for completion. They occur only in young tissues.
Tropisms are named for the stimulus that causes them. For example, gravitropism, also called geotropism, is bending caused by gravity, and phototropism is bending caused by light. Thigmotropism is bending in response to touch. Tropisms may be positive (bending toward the stimulus) or negative (bending away from the stimulus). Studies of tropisms led to the discovery of auxins, the plant hormones that control growth (see Auxin ).
