(a) A growth movement ofa 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
