Photonastic movement of Oxalis triangularis in response to light. At lowered light levels the leaves fold down; timelapse recorded at ~750x actual speed and covering a 1.5 hr period of time.
Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor or changes in growth (therefore K+ ion concentration usually controls such movement is plants). Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position. The rate or frequency of these responses increases as intensity of the stimulus increases. An example of such a response is the opening and closing of flowers (photonastic response). They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:
Epinasty: downward-bending from growth at the top, for example, the bending down of a heavy flower.