(a) What is meant by ‘tropisms’ (or tropic movements) ? Explain with an example. (b) What are the different types of tropisms ?

(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