Question

what is the main difference between tropic and nastic movements in plants?

Answer 1

Both tropic and nastic movements are influenced by environmental cues.

Tropic movements are found virtually in all plants but nastic movements are mostly found in specialized plants and organs.

In tropic movements, there is directional growth of a plant, or part of a plant, in response to an external stimulus such as gravity or light. So the direction of the stimulus controls the orientation of growth. The direction of a nastic movement is independent of where the signal comes from. It is only the quality, and not the direction, of the stimulus that triggers a response.

In tropism, the effect is more or less permanent. Most nastic movements are temporary and reversible.

Nastic movements are more specialized in function and distribution than tropic movements.

Answer 2

In tropic movements, plants show response in a particular direction i.e. towards the stimulus (positive tropic movement) or away from the stimulus (negative tropic movement)
Example: Sunflower facing the Sun.

In nastic movements, stimulus doesn't affect the direction of response.
Example: Leaves of Touch-Me-Not (Mimosa pudica) closing on touch.