The growth of a plant part in response to a water potential gradient. Roots, for example, grow towards water in the soil; i.e. they show positive hydrotropism. The mechanism is not fully understood, but studies in thale cress (Arabidopsis thaliana) show that the elongation zone behind the root tip serves as a moisture sensor and undergoes differential growth to direct the root towards the water. The process involves the plant hormone abscisic acid. Another study in pea seedlings has shown that roots respond to the acoustic vibrations of water flowing in nearby pipes, suggesting a possible mechanism of long-range detection of water sources by plant roots. Other factors, such as gravity and light, also affect root growth, and how these are integrated for optimal direction of growth remains unclear. See geotropism; tropism.