tropism
Directional growth of a plant, or part of a plant, in response to an external stimulus such as gravity or light. If the movement is directed towards the stimulus it is described as positive; if away from it, it is negative. Geotropism for example, the response of plants to gravity, causes the root (positively geotropic) to grow downwards, and the stem (negatively geotropic) to grow upwards.
Phototropism occurs in response to light, hydrotropism to water, chemotropism to a chemical stimulus, and thigmotropism, or haptotropism, to physical contact, as in the tendrils of climbing plants when they touch a support and then grow around it.
Tropic movements are the result of greater rate of growth on one side of the plant organ than the other. Tropism differs from a nastic movement in being influenced by the direction of the stimulus.
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