(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?
(a) The growth of a plant part in response to a stimulus is called positive tropism and if the growth of a plant part is away from the stimulus, then it is called negative tropism.
Example: The roots of a plant go towards earth in response to gravity is an example of positive geotropism whereas stem grows away from earth against gravity is an example of negative geotropism.
(b) The directional growth movement of a plant part in response to the touch of an object is called thigmotropism.
Example: Tendrils grow towards any support which they happen to touch and wind around it.
(c) Thigmotropism: The directional growth movement of a plant part in response to the touch of an object is called thigmotropism.
Example: Tendrils.
Thimonasty: The non-directional movement of a plant part in response to the touch of an objects called thigmonasty.
Example: Mimosa pudica.
Tendrils grow towards a stimulus hence it is a directional movement which shows that it is a case of thigmotropism whereas, the folding of leaves in mimosa plant does not depend on the direction of stimulus (touch) which shows that it is an example of thigmonasty.