Normally, plants are fixed in the soil and do not move here and there. However movement is seen in certain cases. We are going to read about Tropic Movements in Plants.
Chlamydomonas and Volvox show movement by means of flagella. Mosses and ferns are ciliated and move about in water. Plants show movement in response to certain stimuli like-light, water, gravity, wind and touch.
Some of the plant movements that we usually observe are as follows :
Opening and closing of stoma
Flower closing during night
Sunflower turns toward the sun
Movement of insectivorous plants and movement of roots in the soil
These movements can be divided into two categories.
(A) Tropic movements in plants (Tropism)
(B) Nastic movements in plants
(A) Tropic movements (Tropism)
A tropism is the movement of part of a plant either towards the source of stimulus (Positive) or away from it (Negative). These are the movements of curvature brought about by more growth on one side and less growth on the opposite side of plant organ induced by some external stimuli.
Depending upon the nature of stimuli these movements are of following five types:
1. Phototropism :
The movement of plant parts in response to light is called phototropism
. Most of the stems and flower stalks moves towards light and are called positively phototropic movements in plants
Roots are negatively phototropic as they move away from light. The leaves are transversly phototropic as they keep their faces at right angles to the direction of light. Such leaves are called diaphototropic.
2. Geotropism :
The movement of plant parts in response to the force of gravity is called Geotropism. The primary root always grow vertically towards downward direction towards the force of gravity, while shoot moves just opposite to the force of gravity.
Therefore, the root is positively geotropic and the shoot is negatively geotropic. The secondary roots and branches place themselves at right angles to the force of gravity and are called diageotropic. Example, Rhizome and stolon.
It is a special apparatus in which if a plant is rotated neutralizes the effect The shoot continue growing of gravity. Horizontally and does not show any indication of curvature, since rotation causes equal distribution of auxin on all sides of the plant.
Equal distribution of auxin causes equal growth on all sides and no curvature takes place.
3. Hydrotropism :
The movement of plant parts in response to the water is called Hydrotropism. Roots are positively hydrotropic as they bend towards the source of water.
4. Thigmotropism :
Growth / tropic movements in plants
made by plants in response to contact (touch) with a solid object are called thigmotropism.
These are curvature movements and are most apparently seen in tendrils and twiners.Whenever the supporting organ touches a support, it makes a curvature movement to get hold of the object and then tightens its hold.
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.
Example : The plants such as bitter gourd, bottle gourd, grape vine and passion flower have stem tendrils which are positively thigmotropic and make these plants to climb up by winding around various types of support.
The plants such as peas and glory lily have leaf tendrils which are positively thigmotropic. These leaf also make their plants to climb up by winding around various types of nearby support.
5. Chemotropism :
The tropic movements in plants in response to the stimulus of the chemicals are called Chemotropic movements. Examples:
- Growth of pollen tube through the style towards the embryo sac.
- Movement of pollen tube towards ovary due to absorption of calcium and borate from style of carpel.
- Movement of tentacles in Drosera, an insectivorous plant.
- Closing of lid of Nepenthes, an insectivorous plant, due to nitrogenous food.
- Movement of fungal hyphae towards sugars and peptones.
- Penetration of haustoria of parasite into host body.
B) Nastic movements
There are also responses due to stimulus of contact, light, heat etc. In this case movement is not determined by the direction from which the stimulus is applied.
These are non-directional ompared to tropic movements in plants. These movements are fast. These movements are due to turgor changes in the cells.
These are of following types :
- Nyctinastic movements : Seen in Oxalis
- Thigmonastic movements : Seen in insectivorous plants.
- Seismonastic movements : Seen in Mimosa pudica plant (Touch me not plant).
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