To kick off the series on nastic movements, we feature Mimosa pudica, or the sensitive plant.
Nastic movements by plants occur in response to a stimulus, but are independent of the direction of the stimulus. After being triggered, the movements are accomplished either through growth or changes in turgor pressure. Tropic movements, on the other hand, cause directional responses either toward or away from the stimulus (for example, the growth of many roots and shoots in response to gravity). There are several types of nastic movement and they are categorized by the stimulus, such as responses to touch, light, time of day, water, and temperature.
Today’s species has a nastic movement called thigmonasty or seismonasty. This is a movement in response to touch. Thigmonasty in Mimosa pudica is caused by a sudden loss of turgor pressure. The trigger, a touch stimulus to the leaves of this plant, causes a chemical signal to be sent to the parenchymatous motor cells in the pulvini (joint-like thickenings of the leaflets). This leads to a rapid loss of various salts in the motor cells triggering a contraction of the vacuoles within, followed by outward water movement. This decrease in turgor pressure results in the folding of the leaflets. Here is a video of the phenomenon: thigmonasty in Mimosa pudica. It is hypothesized that the wilting response is a defence against herbivory, in which the apparent loss of leaf surface area dissuades would-be herbivores from browsing on the plant.
Because of the response to touch, Mimosa pudica is sometimes known as the sensitive plant (pudica means “bashful, shy or shrinking”) among its many other common names. A member of the legume family, Mimosa pudica was first described in Brazil. It is now found in much of the New World Tropics, growing in sunny, open, often disturbed areas with well-draining soil (it is often a colonizer of recent clearcuts). This annual or perennial herb reaches 50-100cm in height. The prickly stem grows from a taproot surrounded by an extensive fibrous root system with nitrogen-fixing nodules. The alternate leaves are bipinnately compound leaves of 10-25 leaflets. Flowers are pink to lilac in color, and clustered into a globose head. Its fruits are bristly loments, which are 2-5 seeded indehiscent legumes that break into single-seeded segments.