(a) What is meant by ‘tropisms’ (or tropic movements)? Explain with an example.
(b) What are the different types of tropisms? Define each type of tropism. Write the name of stimulus in each case.
(c) How do tropisms differ from nasties (or nastic movements)?
(a) A growth movement of a plant part in response to an external stimulus in which the direction of stimulus determines the direction of response is called tropism.
Example: The bending of plant stem towards light is an example of positive phototropism.
(b) Different types of tropisms are:
(i) Phototropism: The movement of a plant part in response to light is called phototropism. Its stimulus is light.
(ii) Geotropism: The movement of a plant part in response to gravity is called geotropism. Its stimulus is gravity.
(iii) Chemotropism: The movement of a plant part in response to a chemical stimulus is called chemotropism. Its stimulus is chemical.
(iv) Hydrotropism: The movement of a plant part in response to a water stimulus is called hydrotropism. Its stimulus is water.
(v) Thigmotropism: The movement of a plant part in response to a touch stimulus is called thigmotropism. Its stimulus is touch.
(c) Tropisms:
1. These movements are always in the direction of the stimulus.
2. These movements are slow.
3. These movements are exhibited by all parts of a plant. For example, movement of shoot towards the light and not towards
Nasties:
1. These movements are neither away nor towards the stimulus.
2. These movements are fast.
3. These movements are exhibited by the flat organs (like leaves and petals of flowers) of a plant. For example, the bending and dropping of leaves in ‘Touch-me-not’ plant.