An organic compound either natural or synthetic that modifies or controls one or more specific physiological processes within the plant. If the compound is produced by the plant it is called a plant hormone, e.g., auxin, which regulates the growth of longitudinal cells involved in bending of the stem one way or another. Substances applied externally also bring about modifications such as improved rooting of cuttings, increased rate of ripening (ethylene), and easier scission (separation of fruit from stem). A large number of chemicals tend to increase the yield of certain plants such as sugarcane, corn, etc. All these, as well as plant-produced hormones, are included in the term plant growth regulator.See dinitrobutylphenol; Kinin; Gibberellin; Abscisic Acid.