Nastic movements are different from tropic movements in which way?

Nastic movements are the non-directional movements of plant parts in response to stimuli. Example: Mimosa pudica or touch-me-not plant. 

Tropic and nastic movements are both plant responses to external stimuli. however, tropic movements depend on the direction of the stimulus and the nastic movements do not depend on the direction of a stimulus.

Example of nastic movement: Closing of the leaves of touch me not plant on touching.

Example of tropic movement: Growth of the roots in the downward direction.