tropism (trōpˈĭzəm) [key], involuntary response of an organism, or part of an organism, involving orientation toward (positive tropism) or away from (negative tropism) one or more external stimuli. The term tropism is usually applied to growth and turgor movements in plants; an involuntary orientation of a microorganism toward or away from an external stimulus is commonly called a taxic movement, or taxis—e.g., the negative phototaxis of certain protozoans that move away from light. Tropistic stimuli include light, heat, moisture, gravity, electricity, and chemical agents. Plant stems are positively phototropic and negatively geotropic, i.e., they grow toward light and against gravity; roots are the reverse, as well as positively hydrotropic (moisture-seeking). Tropistic growth in plants is believed to be triggered by the presence of plant hormones (see auxin) that promote cell growth. Auxin action is apparently inhibited by light; hence, if a plant is placed in a position of unequal lighting, the cells on the shadier side elongate faster than those on the illuminated side, and the plant bends toward the light. There is also evidence that auxins are affected by gravity, i.e., they accumulate in the lower portions of the plant organs. Since an overconcentration of these hormones inhibits growth, the cells on the underside of a root elongate more slowly than those on the upper side, resulting in the root's downward growth. Generalized plant responses to a stimulus are called nastic movements, or nasties. These include the opening of bud scales and of flower petals, growth movements that occur in response to stimuli such as light and heat without regard for the direction of the stimulus. Some spring flowers exhibit thermonasties, i.e., their flowers open in response to warmth rather than the amount of light. Turgor movements are effected by changes in the water content of cells and are often quite rapid. Examples are the "sleep movements" of clover, the sudden drooping of the leaves of the sensitive plant (mimosa) when touched (thigmotropism), and the reactions of insectivorous plants to the presence of their prey. The exact mechanism controlling the sudden loss of water pressure in certain cells, producing turgor movements, is not clearly understood.