Plant movement

PLANT MOVEMENT

While doing any work our body parts move and also entire body moves from ‘one place to another as per our desire.’ This is called movement of our body. Plants also show movements. But their movements are completely different from our body movement. Except some unicellular plants, all other higher plants cannot move from one place to another place as their roots are fixed in the soil. Still they show movement by folding the buds, opening and closing the flowers, bending towards sun light. etc. These movements in plants are very slow and we have to wait and observe them carefully and patiently to notice these movements. Let us learn about various types of movements shown by plants.

(a) Tropic Movement (directional response or growth movements)

Movement in plants or in any part of the plants towards or away from some environmental factors is known as tropic (trope: turn) movement. You must have observed 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. These are examples of tropic movement.

(i) Phototropism : Induced by light e.g. bending of stems towards light.

(ii) Geotropism : Induced by gravity e.g. growth of roots towards gravity.

(iii) Thigmotropism : Movement caused by contact e.g. twining stem and tendril and the drooping of leaves of sensitive plant by touch.

(iv) Hydrotropism : Induced by water i.e. growth of roots towards source of water.

(b) Nastic Movement

The nastic (nastein : bending) movements are the growth movements resulting due to difference in the rate of growth on opposite sides of an organ e.g. opening of petals, coiling of leaves, etc. When upper side of an organ grows faster than the lower side, the movement is called epinasty. (e.g. downward curling of leaf, opening of sepals of gull mohur flower.) When the lower side grows more rapidly than upper side, it is called as hyponasty. (e.g. 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 example of turgor movements are :

(i) leaves or leaflets of some plants close on the fall of darkness (sleep movement) . Example Portulaca, Acacia.

(ii) Closing of leaflets and drooping of leaves in response to a strong stimulus of blowing wind or of touch. Example – Sensitive plant (Mimosa pudica)

(iii) Closing of leaves of Venus Flytrap to catch a landing insect.

(iv) Seed pods of some plants open on maturity, vigorously expelling their seed. Example – Balsam (Gulmehandi).