Back in September, I quoted a tweet by AsapSCIENCE that displayed the rapid plant movement of the Mimosa pudica, also known as the “sensitive plant”.
I decided to look into these “sensitive plants” a little more, and found that their movement is a nastic response, generated by physical contact or “thigmonasty”. The reason why Mimosa pudica has evolved to have this trait is not exactly known. Some scientists believe that the plant uses its ability to close as a defense mechanism from herbivores because some animals may be afraid of a fast moving plant and would rather consume a less active one. It could also be because abrupt movement dislodges harmful insects.
Nastic movements are plant movements that occur in response to environmental stimuli. Movement could be stimulated in response to light, gravity, alternation between night and day, water, temperature, chemicals or nutrients, and of course, contact. The frequency of this movement will typically increase as the intensity of the stimuli increases. For example, speed of the nastic response generated by Mimosa pudica depends on the magnitude in which you touch it. If you gently graze the leaves, the movement will propagate slowly from the tip of the plant, to the stem. If you flick the plant abruptly, the leaves will close very quickly.This response will also occur with various intensities of heat.
The movement of Mimosa pudica is driven by turgor pressure, which is the pressure exerted on the cell wall because of the movement of water into the cell. The environmental stimuli induces an electrical signal (action potential) transmitted from touch-sensitive cells. The action potential moves from cell-to-cell until it reaches specialized “motor cells” at the base of each leaf-blade or leaflet. From here, movement occurs as the action potential translates into a chemical signal, which causes rapid efflux of potassium (K+) followed by water transport out of the motor cells due to osmosis. The greater the intensity, the farther the signal will travel, resulting in greater movement. As a result, there is a loss in cell pressure and cell collapse causing the leaves to close. The leaves will usually return to their original orientation after 10-15 minutes.
Mimosa pudica is similar to other species of plants that demonstrate rapid plant movement. A famous example would be the carnivorous Venus flytrap (Dionaea muscipula), which snap shut when they detect movement on their surface. A major difference between the two being that the movement of Mimosa pudica has a defensive purpose, whereas the Dionaea muscipula has a predatory purpose. Each half of the Venus flytrap has hair cells, which are extremely sensitive to mechanical stimuli and will generate an action potential. As a result, the plant will shut together on their prey, and slowly digest them with the help of enzymes.
For the sake of more fun gifs, another plant that demonstrates nastic movement is the Squirting Cucumber (Ecballium agreste). These plants spread their seeds via rapid movement, by “squirting” out a liquid containing their seeds once they have reached maturity. Seeds may travel 10-20 feet away from the plant, so watch out!