Explain about tropic movement and its 4 types and also explain about its types
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The six types are: (1) Thigmotropism (Haptotropism) (2) Phototropism (3)Geotropism (4) Thermotropism (5) Chemotropism and (6) Hydrotropism.
Type # 1. Thigmotropism (Haptotropism):
Growth movements made by plants in response to contact with a solid object are called thigmotropism. These are curvature movements and are most apparently seen in tendrils and twiners. In most plants the curvatures of the tendrils which follow contact with a support are mostly the result of increased growth on the side opposite the stimulus
Type # 2. Phototropism:
This kind of movement is induced by light. Not all plants and not all parts respond in the same way to this stimulus. In general, the stem mostly grows and turns towards the source of light, while the roots away from it. As shown in fig. 7.10 the leaves also positively respond toward the source of light. The leaves, however, take up such a position in which the broad surface of the blade is at right angles to the light rays. A stem is, therefore, said to be positively phototropic, a root negatively phototropic, and a leaf transversely phototropic or diaphotropic. Phototropism is also known as heliotropism.
In certain plants, such as Arachis hypogea (ground nut) more complex changes occur within a short period of time. The flower-stalks of this plant initially show positive phototropism until they have produced flowers. Soon after fertilization the stalks curl up and eventually bury the developing pods under the soil, thus showing negative phototropism.
Type # 3. Geotropism
Any reaction to the stimulus of earth’s gravity is called geotropism. The effects of gravity on plants are not like those of light and temperature because it is both continuous in action and constant in strength. Primary roots and certain other portions of the root system tend to grow directly toward the centre of gravity and hence called positively geotropic.
Stem mostly grows away from the centre of gravity and is thus negatively geotropic. However, stems in prostrate plants have lost their negative geotropism and even develop into root stock or tubers which behave exactly like roots. Most of the leaves take up their positions at right angles to the centre of gravity and are, therefore, called transversely geotropic or diageotropic. Geotropism is of three types: orthogeotropism (e.g. primary root), plageotropism (e.g., secondary roots) and diageocropism (movement of tertiary roots)
Type # 4. Thermotropism:
Some of the plant organs markedly respond towards fluctuating atmospheric temperature. In response to this kind of stimulus plant parts exhibit curvature movements in order to take some advantageous position. Such movements are called thermotropism.
Type # 5. Chemotropism:
Certain chemical substances are responsible to bring about curvature movements in plant organs. For instance, movement of Pollen tube towards ovary due to absorption of calcium and borate from style of carpel; movement of tentacles in Drosera, closing of lid of Nepenthes due to nitrogenous food, and penetration of haustoria of parasite into host body etc.
Type # 6. Hydrotropism:
The paratonic curvature movements of growth in relation to the stimulus of water are called hydrotropic movements. The tropic response to the stimulus of water is called hydrotropism. The roots show positive hydrotropic response, i.e., they bend towards the water hydrotropism is stronger in roots compared to geotropism.The exact mechanism of hydrotropic response is not know. Probably roots detect and react to differences in the concentration of water.
Type # 1. Thigmotropism (Haptotropism):
Growth movements made by plants in response to contact with a solid object are called thigmotropism. These are curvature movements and are most apparently seen in tendrils and twiners. In most plants the curvatures of the tendrils which follow contact with a support are mostly the result of increased growth on the side opposite the stimulus
Type # 2. Phototropism:
This kind of movement is induced by light. Not all plants and not all parts respond in the same way to this stimulus. In general, the stem mostly grows and turns towards the source of light, while the roots away from it. As shown in fig. 7.10 the leaves also positively respond toward the source of light. The leaves, however, take up such a position in which the broad surface of the blade is at right angles to the light rays. A stem is, therefore, said to be positively phototropic, a root negatively phototropic, and a leaf transversely phototropic or diaphotropic. Phototropism is also known as heliotropism.
In certain plants, such as Arachis hypogea (ground nut) more complex changes occur within a short period of time. The flower-stalks of this plant initially show positive phototropism until they have produced flowers. Soon after fertilization the stalks curl up and eventually bury the developing pods under the soil, thus showing negative phototropism.
Type # 3. Geotropism
Any reaction to the stimulus of earth’s gravity is called geotropism. The effects of gravity on plants are not like those of light and temperature because it is both continuous in action and constant in strength. Primary roots and certain other portions of the root system tend to grow directly toward the centre of gravity and hence called positively geotropic.
Stem mostly grows away from the centre of gravity and is thus negatively geotropic. However, stems in prostrate plants have lost their negative geotropism and even develop into root stock or tubers which behave exactly like roots. Most of the leaves take up their positions at right angles to the centre of gravity and are, therefore, called transversely geotropic or diageotropic. Geotropism is of three types: orthogeotropism (e.g. primary root), plageotropism (e.g., secondary roots) and diageocropism (movement of tertiary roots)
Type # 4. Thermotropism:
Some of the plant organs markedly respond towards fluctuating atmospheric temperature. In response to this kind of stimulus plant parts exhibit curvature movements in order to take some advantageous position. Such movements are called thermotropism.
Type # 5. Chemotropism:
Certain chemical substances are responsible to bring about curvature movements in plant organs. For instance, movement of Pollen tube towards ovary due to absorption of calcium and borate from style of carpel; movement of tentacles in Drosera, closing of lid of Nepenthes due to nitrogenous food, and penetration of haustoria of parasite into host body etc.
Type # 6. Hydrotropism:
The paratonic curvature movements of growth in relation to the stimulus of water are called hydrotropic movements. The tropic response to the stimulus of water is called hydrotropism. The roots show positive hydrotropic response, i.e., they bend towards the water hydrotropism is stronger in roots compared to geotropism.The exact mechanism of hydrotropic response is not know. Probably roots detect and react to differences in the concentration of water.
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