The directional movement of plant parts towards or away from a stimulus is known as tropic movement or tropism.
Different types of tropic movements in plants:
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Tropic Movement
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Description
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Phototropism
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- Movement of plant parts towards or away from light is termed phototropism.
- Because shoots of most plants grow towards the source of light, it is termed positive phototropism.
- Roots grow away from light and hence are negatively phototropic.
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Geotropism
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- Movement of plant organs in response to gravity is termed geotropism.
- Roots are positively geotropic because they grow in the direction of gravity.
- The shoot grows upwards, i.e. against gravity, and hence is negatively geotropic.
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Chemotropism
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- Movement of plant organs in response to a chemical stimulus is called chemotropism.
- When plant organs grow away from the chemical response, it is called negative chemotropism.
- When plant parts grow towards the chemical response, it is called positive chemotropism. The pollen tube grows towards the sugary substance secreted by the stigma of the flower.
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Hydrotropism
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- Movement of plant organs in response to water is termed hydrotropism.
- Roots grow towards the source of moisture and hence are positively hydrotropic.
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Thigmotropism
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- Movement of plant organs in response to stimuli caused by physical contact with solid objects is termed thigmotropism.
- Weak-stemmed plants use twining stems and tendrils to climb on other plants/objects which provide them support. Hence, twining stems and tendrils are positively thigmotropic.
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