Movements of plants are completely different from body movement. Except some unicellular plants, all other higher plants cannot move from place to place as their roots are fixed in the soil. Still they show movement by folding the buds, opening and closing the flowers, and bending towards sun light. These movements in plants are very slow and you have to wait and observe them carefully and patiently to notice these movements.
Movement in plants or in any part of the plants towards or away from some environmental factors is known as tropic movement. For example, the movement of plants in the direction of light, the downward movement of roots in the soil, drooping of leaves of some sensitive plants by touch, etc.
Phototropism: Induced by light e.g. bending of stems towards light.
Geotropism: Induced by gravity e.g. growth of roots towards gravity.
Thigmotropism: Movement caused by contact e.g. twining stem and tendril and the drooping of leaves of sensitive plant by touch.
Hydrotropism: Induced by water i.e. growth of roots towards source of water.
The nastic movements are the growth movements resulting due to difference in the rate of growth on opposite sides of an organ. For example, opening of petals, coiling of leaves, etc. When upper side of an organ grows faster than the lower side, the movement is called epinasty like downward curling of leaf, opening of sepals of goldmohur flower. When the lower side grows more rapidly than upper side, it is called as hyponasty like upward curling of leaf blade.
These movements are due to change in the volume of water inside the cell. When more water is present in the cell it is fully expanded and becomes rigid or hard. Such a condition is called turgidity and the cell is said to be turgid. When less water is present inside the cell, it is not fully expanded and remains soft. This is called flaccid condition. The leaves bend in hot summer due to excessive transpiration on account of loss of turgidity of cells of the leaf.
Some examples of turgor movements are:
- Leaves or leaflets of some plants close on the fall of darkness (sleep movement). Example - Portulaca, Acacia.
- Closing of leaflets and drooping of leaves in response to a strong stimulus of blowing wind or of touch. Example - Sensitive plant (Mimosa pudica)
- Closing of leaves of Venus Flytrap to catch a landing insect.
- Seed pods of some plants open on maturity, vigorously expelling their seed. Example - Balsam (Gulmehandi).