Tropism
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 right angle, and plagiotropic responses at some oblique angle to the direction of the stimulus.
Tropic responses typically result from shifts in an organism's direction of growth, resulting in a change in its orientation with respect to the stimulus. Phototropism in plant shoots has been especially well studied, and has been found to result from changes in concentration of growth hormone on the side of the plant toward the light and the side away from the light.
Auxin, a plant hormone that stimulates cell elongation, moves to cells on the side of the plant away from the light, causing those cells to expand more than cells closest to the light. This causes the plant to bend toward the light, a phenomenon well known to anyone who has attempted to grow plants in front of a windowpane. Plants in this circumstance have to be rotated regularly to maintain symmetrical growth. Gravitropism may also be explained by a redistribution of growth hormone within the root and stem, but the mechanism is less well understood. Plant shoots tend to grow upward, away from the force of gravity, exhibiting negative gravitropism, while roots grow downward, showing positive gravitropism. Tropisms in animals are found in sessile (i.e. sedentary) forms such as hydra that can direct growth-curvature movements in response to environmental stimuli.
A related phenomenon called taxis is found in animals and microorganisms that are able to move in response to a stimulus. Thus, motile bacteria that swim toward a directional source of light are said to exhibit positive phototaxis, and those that swim away show negative phototaxis. Microorganisms may exhibit a phenomenon called aerotaxis or movement in response to an oxygen gradient. Motile anaerobic bacteria may move away from an increased supply of oxygen showing negative aerotaxis, whereas aerobes may move toward the supply exhibiting positive aerotaxis. Microaerophyllic bacteria, intermediate in their affinity for oxygen, may accumulate between the anaerobes and aerobes in a region low in oxygen.
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