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Concept
Thigmonasty
Summary
In biology, thigmonasty or seismonasty is the nastic (non-directional) response of a plant or fungus to touch or vibration. Conspicuous examples of thigmonasty include many species in the leguminous subfamily Mimosoideae, active carnivorous plants such as Dionaea and a wide range of pollination mechanisms.
Thigmonasty differs from thigmotropism in that nastic motion is independent of the direction of the stimulus. For example, tendrils from a climbing plant are thigmotropic because they twine around any support they touch, responding in whichever direction the stimulus came from. However, the shutting of a venus fly trap is thigmonastic; no matter what the direction of the stimulus, the trap simply shuts (and later possibly opens).
The time scales of thigmonastic responses tend to be shorter than those of thigmotropic movements because many examples of thigmonasty depend on pre-accumulated turgor or on bistable mechanisms rather than growth or cell division. Certain dramatic examples of rapid plant movement such as the sudden drooping of Mimosa pudica or the trapping action of Dionaea or Utricularia are fast enough to observe without time lapse photography; some take less than a second. Speed is no clear distinction however; for example the re-erection of Mimosa leaves is nastic, but typically takes some 15 to 30 minutes, rather than a second or so. Similarly, re-opening of the Dionaea trap, though also nastic, typically takes days to complete.
Botanical physiologists have discovered signalling molecules called turgorins, that help mediate the loss of turgor. In species with the fastest response time, vacuoles are believed to provide temporary, high speed storage for calcium ions.
Thigmonasty other than leaf closure occurs in various species of thistles. When an insect lands on a flower, the anthers shrink and rebound, loading the insect with pollen. The effect results from turgor changes in specialized, highly elastic cell walls of the anthers. Similar pollination strategy occurs in Rudbeckia hirta.
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