Nastic movements occur in response to a stimulus. It is independent of its direction. They are shown by bifacial organs (leaves, sepals, petals) and are in response to a diffused external stimulus.
Seismonastic movements are due to shock or touch stimulus. Such movements are shown by Mimosa pudica. The best example of seismonastic movement is the leaves of sensitive plant Mimosa pudica (Touch me not). It shows both nyctinastic (Sleeping movement) and seismonastic movement (shock movements).
The leaf of Mimosa is compound having four pinnae and each pinna is having many pinnules. If a terminal pinnule is touched, the whole leaf droops down. This is due to the fact that at the bases of the pinnules, pinna and whole leaf, a swollen structure called pulvinus is present and drooping is due to loss of turgidity of the lower portion of pulvinus.