Question 37:
(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 ?
Lakhmir Singh Biology Class 10
Answers
Answer:
(a) Positive tropism is the movement or growth of plants toward the direction of the stimulus. For example, the roots of a plant always grow towards the pull of gravity. Negative tropism is the movement or growth of plants away from the direction of the stimulus. For example, the shoot of a plant always grows in the opposite direction of the gravitational pull.
(b) Thigmotropism is the response of a plant to the physical contact. Thigmotropism is directional which means the plant response towards or away from the direction of the physical contact. For example, the growth of ivy on a wall due to the physical contact with the wall.
(c) Thigmotropism is the directional response of the plant to the physical contact whereas thigmonasty is the non-directional response of the plant to the physical contact. An example of thigmotropism is tendrils of grapevine. The tendrils grow towards the object with which it experiences a physical contact. This is a directional response of the tendrils toward the physical contact with the object. An example of thigmonasty is Mimosa pudica. On physical contact, the leaves of Mimosa pudica shrinks and folds. It does not depend on the direction of the physical contact.
(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)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.