The New Student's Reference Work/Rheotropism
Rheot′ropism, the sensitiveness of a plant to the direction of a current of water, to which it responds by changing the rate of growth, which results in a curvature with or against the current. When roots of corn or vetch are grown in moving water, the rate of growth is hastened on the side not struck by the current and slowed on the other side; consequently the tip is turned up-stream. A very slow current will produce the reaction. The exact nature of the stimulus has not yet been determined. See Irritability.