Plants move in response to a stimulus. The plant growth can be divided into:
- Tropic Movements
- Nastic Movement
Growth movements which occur in the direction of the stimulus are known as tropic movements. In this, the response acts on the protoplasm from one side. A tropic movement may be towards or away from the stimulus.
Tropic movements are of various types:
Phototropism: Response to light
In this, the plant grows in the direction of light. The plants have a chemical called auxin, which reacts in response to light. This causes the cells of the plants to elongate. Growth towards light is positive phototropism, while that away from the light is negative phototropism. The stem grows towards the direction of light and is called positively phototropic. However, the root is called negatively geotropic since it grows away from the light source.
Chemotropism: Response to chemicals
It is the movement of the plant part towards a chemical stimulus. For eg., during fertilization in plants, the stigma produces a chemical in response to which the pollen tube grows towards the ovule.
Hydrotropism: Response to water
It is the growth of the plant in the direction of the water gradient. For eg., the plant roots grow in the direction of higher humidity level. That is why, when we pull a plant we find its roots growing in all the directions.
Geotropism: Response to gravity
It is the movement of plant parts in response to gravity. The roots grow towards gravity and are hence called positively geotropic. The stem, on the other hand, is negatively geotropic.
Thigmotropism: Response to touch
The growth movement regulated by touch is known as thigmotropism. This is usually found in twining plants and tendrils.
Thermotropism: Response to temperature
The growth movement regulated by temperature is known as thigmotropism. The peduncles of Anemone nemerosa grows in response to the heat imparted by the sun.
These movements are non-directional responses to the stimuli. These movements are independent of the direction of the stimulus. These movements can be due to changes in turgor or growth. For eg., the response of the touch-me-not (Mimosa pudica) plant to touch. However, this is not a growth phenomenon.
Thus, we see that the tropic movements are growth-dependent while the nastic movements are growth independent.
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