Question

(a) What is meant by positive tropism and negative tropism? Explain with examples.
(b) Define thigmotropism. Give one example of thigmotropism.
(c) What is the difference between thigmotropism and thigmonasty? Name one plant which exhibits thigmotropism and one plant which exhibits thigmonasty. What behaviour (or responses) of these plants make you think that they exhibit thigmotropism and thigmonasty respectively?

Answer 1

(a) If the movement of the response is towards the stimulus, it is known as positive tropism; whereas if the movement of the response is in the direction opposite to the stimulus, it is is known as negative tropism.
The movement of roots towards the pull of gravity is an example of positive geotropism, whereas the movement of plant stem in the direction opposite to the pull of gravity is an example of negative geotropism.

(b) The directional movement of plant parts in response to touch is known as thigmotropism. The movement of tendrils of climbing plants towards the things they happen to touch is an example of thigmotropism.

(c)

Thigmotropism	Thigmonasty
It is a directional movement of plant parts in response to touch.	It is a non-directional movement of plant parts in response to touch.
Example: Bitter gourd	Example: Mimosa pudica

Bitter gourd exhibits thigmotropism as its tendrils show directional movement towards the things they happen to touch.
On the other hand, Mimosa pudica exhibits thigmonasty as its leaflets show non-directional movement in response to touch.