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| Induced or Paratonic Movements |
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| Tactic movements |
| These movements are a result of the response to an external stimuli and the direction of movement is controlled by the direction of the stimulus. It is of the following type Photonastic response to light |
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| Thermonastic response to temperature |
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| Chemonastic response to chemicals |
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| Tropic Movements |
| Tropic movements are movements caused by an external stimulus coming from one direction only. |
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| Depending upon the nature of the stimulus there are various types of tropic movements. The chief stimuli are gravity, light, water and chemical substances. The movements induced by them are geotropism, phototropism, hydrotropism and chemotropism respectively. |
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| The movement of stems and roots in response to the force of gravity is called geotropism. Roots move towards gravitational pull and are positively geotropic. Stems move opposite to the force of gravity and are negatively geotropic. |
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| Secondary roots and branches grow at right angles to the force of gravity and are said to be plageotropic. Lateral roots grow oblique to the force of gravity and are known as diageotropic. Tertiary roots are not affected by gravity and are said to be ageotropic. |
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| Take 4 bean seeds and keep them in different positions on moist cotton. Observe them after a few days. |
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| It will be seen that after a few days all the seeds have germinated in different positions. In all the cases the roots are found growing in the downward direction and shoots in the upward direction. |
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| Take a pot with a young plant and rest it on its side so that the stem lies horizontally. Observe after a few days. |
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| Geotropism in Shoots |
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| It is seen that the shoots are found growing upwards after showing a curvature. This is due to unequal distributions of auxin and hormones. A greater concentration of hormones on the lower side promotes greater growth on that side and the stem grows upwards. |
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| But in roots, the greater concentration on lower side inhibits growth and the low concentration on the lower side favours the bending of roots downwards towards gravity. |
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| Geotropism in Roots |
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| If a plant is rotated on a special apparatus called clinostat, it neutralizes the effect of gravity and the shoot continues to grow horizontally without any curvature. |
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| Clinostat, An Instrument to Eliminate the Effect of Gravity |
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| The movement of plant organs in response to unilateral effect of light is known as phototropism. Stems are positively phototropic and roots are negatively phototropic. Leaves are transversely phototropic as they keep their faces at right angles to the direction of light. |
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| Take a well watered plant and keep it in a dark chamber, into which light enters by means of a tiny opening. Leave the plant in the chamber for a few days and then observe. |
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| It will be seen that the shoot is found bending towards the source of light, showing that shoot is positively geotropic. |
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| The movement of an organ of a plant in response to the stimulus of water is known as hydrotropism. Roots are positively hydrotropic. |
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| Positive Hydrotropic Response Shown by Root |
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| Tropic movements in response to the stimulus of chemicals is called chemotropic movements. |
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| Example: Growth of pollen tube through the style towards the embryo sac. |
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