Although anchored in place by root systems, plants move their organs in response to many kinds of external stimuli. These movements are called nastic movements and differ from tropic movements in that they are not directed toward or away from the stimulus. Movements triggered by touch, such as closing the traps of insectivorous plants, are called thigmonastic or seismonastic movements. The changing daily cycles of light and darkness produce “sleep” ( nyctinastic) movements in leaves of many species. Most of the actual nastic movements can be explained by changes in the turgor pressure of specially located parenchyma cells after a stimulus has been received.
The growth response to generalized mechanical disturbances is called thigmomorphogenesis. Plants in their natural environment are subject to all manner of jarring, touching, and shaking by the wind, passing animals, rain, and the like. The general response of most plants to such disturbances results in decreased height, increased diameter, and more supportive tissues in the shoots. The change in the general form of the plant apparently results from activation of genes, one of which carries the code for calmodulin, a calcium‐binding protein. Undoubtedly, the thigmomorphic response mechanism is similar to other calmodulin/growth responses of plant cells. Ethylene, important in growth regulation, also is important here.
Some plants move their leaves and flowers toward the sun and track its movement from east to west during the day. The common sunflower got its name because of this trait. In the morning all of its flowers face the east, at noon they lie horizontally facing the zenith, while in the late afternoon and evening they face west toward the setting sun. The phenomenon is called heliotropism, although it is not a true tropic response since it does not involve growth. Turgor pressure changes in parenchyma cells account for the movements.