Thigmonasty or seismonasty is the nastic response of a plant or fungus to touch, heat or vibration. It differs from thigmotropism in that it is independent of the direction of the stimulus. For example, the growth of roots through soil is thigmotropic because the roots grow away from rocks. However, the shutting of a venus fly trap is thigmonastic. The time scale of thigmonastic responses tends to be faster than thigmotropism because thimonasty depends on turgor and bistable mechanisms rather than growth. Certain dramatic examples such as the sudden drooping of Mimosa pudica are fast enough to observe without time lapse photography.
The most spectacular display of thigmonasty occurs in the venus fly trap. When an insect lands on a trap formed by a joined pair of curved leaves, the trap rapidly switches from an open to a closed configuration. Investigators have observed an action potential and changes in leaf turgor that accompany the reflex.
Mimosa pudica is a plant with compound leaves that has attracted detailed investigation. It appears that contact or injury causes leaflet deformation that in turn triggers an action potential. The action potential travels through the plant until it reaches a pulvinus at the base of the leaflet or petiole.
The pulvinus is a motor structure consisting of a rod of sclerenchyma surrounded by collenchyma. The structure is widespread in the legume family. In its extended position, the cells of the entire collenchyma are distended with water. On receiving the action potential signal, the cells in the lower half of the pulvinus respond by expelling potassium and chlorine ions and taking up of calcium ions. This results in an osmotic gradient that draws water out of the affected cells. The lower pulvinus cells temporarily shrink due to water loss. This forces the entire structure to curve down in the manner of a fan. In this contracted position, the pulvinus no longer functions as a support and the petiole droops. In addition, botanists 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.
Many other members of the legume order display the same talent of rapid leaf closure motion in response to touch. These include the telegraph plant and the silk tree. The pea vine closes its leaves around a support in a thigmonastic gesture. Thigmonasty is especially prevalent in the mimosa genus. Catclaw Brier, a prairie mimosa, native to North America, shuts its leaves on contact. Since the plant is attractive to herbivores, its behavior provides protection against grazing.
Sensitive leaves also occur in plants of the sorrel family. Examples include wood sorrel, a pretty, woodland spring flower; Biophytum sensitivum, a palmlike plant from Africa and star fruit, named for its oddly structured fruit.
A different form of thigmonasty than leaf closure occurs in thistle. When an insect lands on a flower, the anthers shrink and rebound. The effect is to load the insect with pollen. Turgor change acts on the specialized, highly elastic cell walls of the anthers to produce the effect. Similar behavior has been reported in black-eyed susan.
Some fungi exhibit trap closure similar to the venus fly trap. Mycologists have discovered action potentials in fungi but it is not currently known if they are related to thigmonasty.