Responsive Growth Movements: TropismsResponsive growth movements toward or away from an external stimulus are called tropisms. If the plant movement is toward the stimulus, it is a positive tropism;away from the stimulus, a negative tropism. PhototropismThe tropic response to unidirectional light is called phototropism. In general, shoots grow toward light and hence are positively phototropic; roots grow away from light and are negatively phototropic. Well-known and often-repeated experiments with oat seedlings have shown that the auxin IAA, which causes elongation of cells, migrates to the shaded side of oat coleoptiles. The subsequent differential growth on the two sides causes the coleoptiles to bend toward the light. Although green stems also bend and grow toward the light, in this case an IAA inhibitor prevents cells from
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