Thigmonasty is the scientific term to describe plants that exhibit a real-time observable touch response. These plants actually move when you touch them, most commonly in a closing up motion. Many different varieties of plants show thigmonastic responses. However, thigmonastic plants do not always exhibit these responses for the same reason other plants do.
Mechanisms of Closure
Thigomonastic plants have specialized structures that enable them to make their rapid movements. The primary method of action for some is through turgor. Turgor refers to water pressure. When a thigomonastic response is triggered, whether through the stimulation of trigger hairs or receptors on the plant surface, a chemical action potential is initiated. That action potential travels to vacuole reservoirs of water. There is an ionic depolarization across the surface of the vacuole and water either is withdrawn or released into plant cells, causing the cells either to lengthen or shorten. This rapid change in turgor pressure on a large scale produces movement.
An alternate mechanism of closure is effected through a structure known as a pulvinus. The pulvinus is an organ that is dedicated to motor response and can be likened to a "muscle" for the plant. It contains a flexor that will shorten upon stimulation of an action potential and drive movement. It is found in plants, like mimosa, and other members of the pea family.
Carnivorous Thigmonastic Plants
Some thigmonastic plant movement responses are designed to enable the plant to hunt prey. These plants can and do photosynthesize, but prey on organisms to supplement nutritional lacks in their environment. The most famous of these carnivorous plants is the Venus flytrap. This plant produces specialized leaves that form two lobes of the "mouth." These lobes are spiked along their outer edges, and have three specialized hairs on their inner surface. When two or more of these inner hairs are touched, the lobes move closer together, trapping the insect in between the lobes and spikes. The Venus flytrap then releases its digestive juices and dissolves the insect over time. A plant that has a similar mechanism of action is the aquatic waterwheel plant.
In contrast, sundew plants have tentacles that have sticky spikes that adhere to the insect being trapped. When an insect is stuck to these spikes, this stimulates the pulvinus located within the tentacle, which will either fling the insect further into the trap, or will wrap the tentacle around its prey to bring more spikes closer into contact with the insect.
Defensive Thigmonastic Plants
The mimosa plant does not prey on other organisms. Instead, its thigmonastic movements are performed for defensive reasons. When touched, the leaves of the mimosa plant contract until they close and lie flat along the stem. The mimosa plant is interesting particularly in that its thigomonastic responses will show gradations depending on the force of the touch. Light touches will cause only slight contractions or the contraction of only the leaves touched. A slightly stronger touch will cause complete closure of the leaf and responses from neighboring leaves. Touches that are strong enough to cause damage to the leaf will cause leaves on the opposite side of the plant to close. These responses serve to protect the mimosa from pests and animals that feed on the mimosa. Another plant with similar reactions is the telegraph plant, or the Machaerium arboreum vine.
Other Thigmonastic Plants
Though carnivorous and defensive responses are the most common reasons for closing up when touched, some thigmonastic responses serve other functions. Some vining plants have tendrils whose tips also are touch sensitive. These tendrils will retract upon touch stimulation, initiating a coiling thigmonastic response that sometimes is very rapid. This coiling response allows the vine to affix itself to a support by closing around it to assist with growth. The trigger-plant exhibits thigmonastic movement that enhances its reproductive success. The anthers of the trigger-plant exist in a default "cocked" position. When an insect lands on the blossom of the trigger-plant, it stimulates the anther, which fires and lands on the insect, depositing a load of pollen on its body.