Tropisms and Nastic Movements

Tropisms

In tropisms, the growth of a plant organ is oriented by an environmental signal. Usually the organ grows toward or away from the stimulus. The former is considered to be a positive tropism, whereas growth away from a stimulus is a negative tropism. Houseplants on a windowsill grow toward the light by positive phototropism. Stems that emerge from seeds buried in the soil grow upward away from gravity by negative gravitropism. Tropisms can also occur at other angles with respect to a stimulus. Modified stems, called rhizomes, grow along the surface of the soil at right angles to gravity, such as in iris plants. Phototropism and gravitropism are by far the most important and widespread of tropisms in plants. In some plants and organs, other physical stimuli, including touch, temperature, and water, can orient growth as well.

Tropisms allow plants to adjust the direction of growth when their environment changes. For example, when a seedling is turned on its side, the root grows gradually downward creating a curvature, an example of positive gravitropism. This occurs in the growing region of the root, a region located close to the root tip.

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