The first type has no detectable extern cause while the latter are termed movements caused by stimulation. The reactions of each group can be either positive or negative. Movements are, too, classified according to the cause of stimulation. Tactic movements are oriented towards a certain direction. Classic examples are phototropism and geotropism .
Nastic movements are independent of the direction of the controlling signal.
An example is the seismonasty of the mimosa. The folding up of the leaflets and the leaves occurs not in the direction of the haptic stimulus. Tactic movements are based on free directional movements that occur either in the same or in the opposite direction of the source of stimulation. In order to understand plant movements is a clear separation between the cause of stimulation, the transmitting of information and the movement itself necessary. The molecular mechanisms of movements are far from being completely understood, but it seems that explanation is not far away anymore.