Why Does the Mimosa Plant Shy Away? Latest Explanation of the Cause

Why does the mimosa plant shy away? Since plants differ from animals, lacking a nervous system and muscles, they cannot perceive external stimuli. But the mimosa plant differs from ordinary plants: when touched by the outside world, its petioles droop and its leaflets close. People interpret this action as “shyness”, which is why it is called mimosa, zhixiu grass, or pachou grass.

The cells of the mimosa plant are supported by a fine, net-like protein called “actin” (that is, the pulvinus contractile body). When closing movement occurs, the phosphate on the actin falls off. As long as the mimosa 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, causing water to escape, which leads to the closing movement. Actin is generally found in the muscle fibers of animals and is related to muscle contraction and relaxation. That it also exists in the mimosa plant is quite rare.

Plant movement is usually caused by changes in turgor pressure within cells. Most mature plant cells have a large vacuole. When the vacuole is filled with water, it presses against the surrounding cytoplasm, making it press tightly against the cell wall and giving 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 vacuole concentration increases, osmotic pressure increases, and water diffuses from outside the cell into 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 close, and the entire leaf droops as if weak and listless. The whole action is completed in just a few seconds. The leaves and petioles of the mimosa plant have a special structure. At the base of the petiole and the base of the leaflets of the compound leaf, there is a relatively swollen part called the pulvinus. The pulvinus is most sensitive to stimuli. Once the leaf is touched, the stimulus is immediately transmitted to the pulvinus at the base of the petiole, 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 petiole, and the entire petiole 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 are filled with parenchyma tissue containing 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, reducing the turgor pressure of the cells in the upper half of the pulvinus, while the intercellular spaces of the parenchyma cells 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 studies have shown that within 0.08 seconds after the mimosa plant is stimulated, the leaves close. After stimulation, the speed of transmission is also very fast, reaching a maximum speed of 10 cm per second. After stimulation, after a short period, everything slowly returns to normal, the leaflets unfold again, and the petiole stands up again. The recovery time is generally 5 to 10 minutes. However, if we continue to tease it and stimulate its leaves repeatedly, it develops a feeling of “boredom” and no longer responds 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 in 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 leaflets and droops its petiole to avoid damage from the violent wind and rain. 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 petiole and the leaflet stalk there is a relatively swollen part called the “leaf cushion”. The leaf cushion 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 while transporting oxygen ions out of the cell, maintaining a certain potential difference between the cell membrane and the neighboring area, called the resting potential. When an external stimulus exceeds a certain limit, this differential permeability suddenly changes, and positively charged calcium ions flood into the cell in large amounts, while potassium ions move in the opposite direction, raising the membrane potential and even making it positive, thereby 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 instantly, so that the cell loses turgor pressure and becomes limp. Therefore, when the leaf cushion at the base of the leaflet stalk is stimulated, the turgor pressure of the parenchyma cells in the upper half of the leaf cushion 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 petiole. The vascular bundles in the petiole merge into a large duct at the leaf cushion to hold the water discharged by the leaf cushion.

The research team investigating why the mimosa plant “shies away” has uncovered the mystery of the mimosa's closing movement: the cells of the mimosa plant are supported by a fine, net-like protein called “actin” (that is, the pulvinus contractile body). When 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, causing water to escape, which leads to the closing movement. Actin is generally found in the muscle fibers of animals and is related to muscle contraction and relaxation. That it also exists in the mimosa plant is quite rare.