A group of naturally occurring, organic substances that influence plant physiological processes at low concentrations. The processes consist mainly of growth, differentiation, and development, although other processes, such as stomatal movement, may also be affected. Typical effective concentrations (in the range of 1 μM externally, or about 1–100 nM internally (about one part per billion), depending on the compound) are far below those where either nutrients or vitamins would be effective.