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tropism(trōp`ĭzəm), 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 taxistaxis
, movement of animals either toward or away from a stimulus, such as light (phototaxis), heat (thermotaxis), chemicals (chemotaxis), gravity (geotaxis), and touch (thigmotaxis). The turning movements of plants in response to stimuli are called tropisms.
..... Click the link for more information. —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 auxinauxin
, plant hormone that regulates the amount, type, and direction of plant growth. Auxins include both naturally occurring substances and related synthetic compounds that have similar effects. Auxins are found in all members of the plant kingdom.
..... Click the link for more information. ) 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.
the movement of plant organs in response to the unilateral action of light, gravity, and other environmental factors as a result of the more rapid growth of cells on one side of a shoot, root, or leaf.
The basis of tropisms and of nastic movements as well is the phenomenon of irritability. The process begins with the plant’s perception of an external stimulus that induces a physiological difference between the two sides of a plant organ. Next there occurs the transmission of a signal to which there is a reaction—a bending caused by the uneven growth rate of the two sides of the organ. The hormonal theory of tropisms, best demonstrated in phototropism and geotropism, is most widely accepted. An oat sprout, illuminated on one side, bends toward the source of light, because its illuminated side grows more slowly than the shaded side. The auxin content turns out to be greater on the shaded, more rapidly growing half; in other words, the phototropic bend is the result of uneven distribution of auxin. In a horizontal stem, auxin accumulates in the lower part, leading to intensified growth of that part of the stem and to an upward bend (positive geotropism). In a horizontal root, auxin also concentrates in the lower part, but the excess of auxin inhibits the growth of the root cells, which are sensitive to auxin. As a result, the root bends down ward (negative geotropism).
V. I. KEFELI