Most plants, and some fungi, microorganisms, and lower forms of animal life, respond to stimuli that act with greater intensity from one direction than another by orienting themselves with respect to the direction in which the stimuli are acting. These responses, called tropisms, may involve one of several kinds of stimuli and are named accordingly. Thus phototropism refers to a response to a directional source of light. Gravitropism, also called geotropism, is a response to gravity. Chemotropism is a response to a source of chemical stimulation, and thigmotropism is a response to mechanical stimulation. Other kinds of tropism have been described including galvanotropism, or electrotropism (response to electric current), hydrotropism (response to water), and traumatotropism (response to a wound lesion). Tropic movements are usually directed toward or away from the source of the stimulus and are called positive or negative orthotropic responses, accordingly. Diatropic responses are at a...