Why Does the Mimosa Plant Feel Shy? An Analysis of the Leaf-Closing Mechanism

Why does the mimosa plant feel shy? Unlike animals, plants have no nervous system and no muscles, so they cannot perceive external stimuli. The mimosa plant, however, differs from ordinary plants: when touched by the outside world, its leaf stalks droop and its leaflets fold shut. People interpret this action as "shyness," which is why it is called mimosa, the shy plant, or the bashful plant.

The cells of the mimosa plant are supported by a fine, net-like protein called "actin" (namely the pulvinus contractile body). When the closing movement occurs, the phosphate on the actin falls off; as long as the mimosa plant absorbs a compound that prevents the phosphate from falling off, it will not change even when touched. Professor Tsuchiya points out that when the actin bundles disperse, the cells are destroyed, and as a result water escapes, which brings about the closing movement. Actin is generally found in the muscle fibers of animals and is related to muscle contraction and relaxation. It was unexpected to find that it also exists in the mimosa plant, which can be called quite rare.

The movement of plants is usually caused by changes in turgor pressure within cells. Most mature plant cells have a very large vacuole. When the vacuole is filled with water, it presses on the surrounding cytoplasm, making it press tightly against the cell wall, and gives the cell wall a kind of pressure that makes the cell firm and swollen, like a fully inflated balloon. The concentration of organic and inorganic substances contained in the vacuole determines the level of osmotic pressure, and the level of osmotic pressure can determine the direction of water diffusion. When the concentration in the vacuole increases, the osmotic pressure increases, and water diffuses from outside the cell to inside the cell and enters the vacuole, increasing the cell's turgor pressure and making the cell swell; conversely, the cell shrinks. This process can only cause slow movements, such as the opening and closing of stomata.

If the leaves of the mimosa plant are touched, they immediately fold up. The greater the force of the touch, the faster they fold, and the entire leaf droops, looking listless. The whole action is completed in just a few seconds. The leaves and leaf stalks of the mimosa plant have a special structure. At the base of the leaf stalk and at the base of the leaflets of the compound leaf, there is a relatively swollen part called the pulvinus. The pulvinus is the most sensitive to stimuli. Once the leaf is touched, the stimulus is immediately transmitted to the pulvinus at the base of the leaf stalk, causing the two leaflets to close. With a slightly greater touch, it is transmitted not only to the pulvinus of the leaflets but also quickly to the pulvinus at the base of the leaf stalk, and the entire leaf stalk droops. Why does this happen? This is because there is a large vascular bundle in the center of the pulvinus, and around the vascular bundle it is filled with parenchyma tissue that has many intercellular spaces. When the vibration is transmitted to the pulvinus, the cell sap in the parenchyma cells of the upper half of the pulvinus is discharged into the intercellular spaces, lowering the turgor pressure of the cells in the upper half of the pulvinus, while the intercellular spaces of the parenchyma in the lower half still maintain their original turgor pressure. As a result, the leaflets stand upright and the two leaflets close, or even the entire leaf droops. Some people have done research showing that within 0.08 seconds after the mimosa plant receives a stimulus, its leaves close. After stimulation, the speed of transmission is also very fast, reaching a maximum speed of 10 cm per second. After the stimulation, a short time later, everything slowly returns to normal, the leaflets unfold again, and the leaf stalk stands up again. The recovery time is generally 5 to 10 minutes. However, if we continue to tease it and stimulate its leaves repeatedly without stopping, it develops a feeling of "annoyance" and no longer reacts at all. This is because continuous stimulation causes the cell sap in the pulvinus cells to be lost and unable to be replenished in time.

This special ability of the mimosa plant has a certain historical origin. Its original home is Brazil in tropical South America, where there are often strong winds and heavy rains. Whenever the first drop of rain hits the leaf, it immediately closes its leaves and droops its leaf stalk to avoid the damage caused by violent storms. This is an adaptation to changes in external environmental conditions. In addition, the movement of the mimosa plant can also be seen as a form of self-defense: as soon as an animal touches it, it folds its leaves, and the animal no longer dares to eat it.

At the base of both the leaf stalk and the leaflet stalks there is a relatively swollen part called the "pulvinus." The pulvinus is very sensitive to stimuli, and in its central part there are many parenchyma cells. When at rest, these cells transport negatively charged chloride ions into the cell and transport oxygen ions out of the cell, keeping a certain potential difference between the cell membrane and the neighboring area, called the resting potential. When an external stimulus exceeds a certain threshold, this differential permeability suddenly changes, and a large amount of positively charged calcium ions floods into the cell, while potassium ions move in the opposite direction, increasing the potential inside the membrane, even making it positive, thus generating an action potential. This phenomenon is called depolarization. The action potential is transmitted. When the cell reaches the action potential, that is, when depolarization occurs, the differential permeability of the cell membrane disappears, causing the water originally stored in the vacuole to be discharged in an instant, so that the cell loses turgor pressure and becomes limp and soft. Therefore, when the pulvinus at the base of the leaflet stalk is stimulated, the turgor pressure of the parenchyma cells in the upper half of the pulvinus decreases, while the parenchyma cells in the lower half still maintain their original turgor pressure, causing the leaflets to stand upright along the direction of the leaf stalk. The vascular bundle in the leaf stalk forms a large channel in the pulvinus to hold the water discharged by the pulvinus.

The research group that studied the reason why the mimosa plant is "shy" uncovered the mystery of the mimosa plant's closing movement: the cells of the mimosa plant are supported by a fine, net-like protein called "actin" (namely the pulvinus contractile body). When the closing movement occurs, the phosphate on the actin falls off; as long as the phosphate does not fall off, it will not change even when touched. Professor Tsuchiya points out that when the actin bundles disperse, the cells are destroyed, and as a result water escapes, which brings about the closing movement. Actin is generally found in the muscle fibers of animals and is related to muscle contraction and relaxation. It was unexpected to find that it also exists in the mimosa plant, which can be called quite rare.