(a) What is meant by nastic movements in plants? Give one example of nastic movements in plants.
(b) What is the difference between photonasty and thigmonasty?
(a) The movement of a plant part in response to an external stimulus in which the direction of response is not determined by the direction of stimulus is called nastic movement.
Example: The folding up of the leaves of a sensitive plant on touching is an example of thigmonasty.
(b) Photonasty:
The non-directional movement of a plant part (usually petals of flowers) in response to light is called photonasty.
Example: The opening and closing of petals of dandelion flowers in response to the intensity of light.
Thigmonasty:
The non-directional movement of a plant part in response to the touch of an object is called thigmonasty.
Example: In mimosa pudica plant, the leaves fold up in response to touch.