life processes

Tropic movements are movements caused by an external stimulus coming from one direction only. Such movements are dependent on the direction of the stimulus. Depending upon the nature of the stimulus there are various types of tropic movements. The chief stimuli are gravity, light, water and chemical substances.   

Example - Movement of plant roots towards the soil and away from light.

 

Nastic movements are non-directional responses to stimuli. Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus' position. For examle - The opening and closing movements of many flowers, and the responses of leaves to changes of temperature and light, are examples of nastic movements.