Nastic Movements 

You might ask, "But what about the Venus flytrap? Is that some sort of tropism?"

When the trap snaps shut on a meal, the plant is certainly responding to a stimulus. However, the direction of response (closing of the leaves) does not depend on the direction of the stimulus (direction of the fly).

The flytrap movement is called a nastic movement. Nastic movements are defined as plant movements caused by an external stimulus, but that are independent of the direction of the stimulus. Another example of a nastic movement is the response by the leaves of a Mimosa plant to touch or mild heat.