Merriam-Webster Collegiate Dictionary - tropism

 noun combining form  Etymology: International Scientific Vocabulary, from trop- tropism heliotropism TROPISM  noun  Etymology: International Scientific Vocabulary -~  Date: 1899  1.  a. involuntary orientation by an organism or one of its parts that involves turning or curving by movement or by differential growth and is a positive or negative response to a source of stimulation  b. a reflex reaction involving a ~  2. an innate tendency to react in a definite manner to stimuli; broadly a natural inclination ; propensity encouraged his ~ toward the theatrical — John Updike  • tropistic adjective