How do nastic movements differ from tropic movements?

Nastic movements and tropic movements are two types of plant movements. 

• Tropic movements are directional movements of a plant in response to a directional stimulus, such as gravity or light. 
• Nastic movements are nondirectional movements of a plant in response to a non-directional stimulus, such as changes in temperature or touch. 

Some nastic movements can vary in degree and direction and can be triggered by chemical stimuli but these do not define the characteristics of nastic movements.