Mimosa pudica bends upon being touched. This occurs due to changes in the turgor pressure in its cells. The behavior is a predator avoidance mechanism.
Mimosa pudica is a perennial herb of the Fabaceae pea family and is native to Central and South America. Also known as the touch-me-not plant or the sensitive plant (also the ‘tickleMe plant’), it is well-known for closing its leaves or folding its leaves inwards when touched.
When it comes to playing with plants, there’s no greater pleasure than touching the leaves of the touch-me-not plant and watching them recoil, shrink, and “go to sleep”. I used to do this a lot as a kid, and still do, whenever a chance presents itself. As an adult, I have often wondered what causes such a response from the plant and how it manages to do this.
The Touch-Me-Not Plant: Mimosa Pudica
Though there are many colloquial names for this unique plant, botanists and the rest of the scientific community call it Mimosa pudica. It is a perennial herb of the Fabaceae family, in the genus Mimosa, and it is native to Central and South America.
Mimosa pudica isn’t the only Mimosa plant to show ‘rapid plant movement’, it is simply the most popular. In 300 B.C., the Greek philosopher Theophrastus mentioned a plant that would ‘wither up and collapse, and then after a time come to life again and flourish’. The plant Theophrastus was referencing was most certainly part of the Mimosa genus, and might have been the species Mimosa asperata, according to some research. These rapid movements aren’t unique to Mimosa species. Carnivorous plants like the ‘Venus fly trap’ employ rapid movements to capture their prey. Species like the cactus Lophophora williamsii and the plant Cornus canadensis spread their pollen by shooting their stamens out when they experience touch. The plants in the Cardamine species have seeds that explode when touched.
These are called nastic movements—movements that happen in response to a stimuli. If the stimuli is touch or contact stimuli, like the wind, then it is called Seismonastic Movements or Thigmonasty. If the stimuli is heat, they are called thermonastic movements. If the reaction is to light, the movements are photonastic, while day- and night-based movements are called nyctinastic movements.
Why do Mimosa pudica leaves close when touched?
Mimosa pudica doesn’t only respond seismonastic movements; it also closes upon being heated. When the plant is exposed to such stimuli, it begins to undergo a series of biochemical and bioelectrical changes that allow it to bend inwards.
The movement originates at the pulvinus, pad-like swellings of petioles and stems that have the ability to twist, which is what allows them to move. The pulvinus has two types of cells, the extensor cells and the flexor cells, which are located in opposition to one another. When the plant experiences a touch, the extensor cells flex, while the flexor cells stretch, jointly creating the movement.
This bending occurs due to a change in the turgor pressure of the extensor cells. The turgor pressure is the force that the water within the cells exerts on the cell walls, helping the cells maintain their shape.
The stimulus (touch) leads to potassium (K+) ions and chlorine (Cl–) ions exiting the cells, mostly likely through ion pumps— protein channels within the cell membrane that allow the transport of ions through the membrane. The reduced concentration of ions from within the cells and the subsequent increase outside creates a chemical gradient leading to water molecules exiting the cell through osmosis. This process might be quickened by the presence of aquaporins—protein channels that allow water to freely pass through the cell membrane.
Water leaving the extensor cells causes a decrease in the turgor pressure, which makes the cells flaccid. This flaccidity is what causes the extensor cells to bend.
However, these aren’t the only changes that occur. Another potentially simultaneous mechanism might be the release of sucrose from the phloem into the apoplast, which is the extracellular region in a plant. This also encourages osmosis to occur.
Within the extensor and flexor cells, there is evidence that the cytoskeleton, the protein chains that help the cells maintain their shape, also undergoes changes to allow the cells to flex.
A stimulus can be propagated throughout the branch through an electrical signal, similar to those that occur in a neuron. So, even though one part of the branch has experienced the stimulus, all the leaves on that branch would still shut in response.
The exact mechanism of how such electrical signals are propagated in plants hasn’t been fully elucidated. Some research has shown that the signal might travel though companion cells and phloem cells within the plant, while other research has implicated the xylem and changes in its pressure as a contributor to the distribution of the stimuli.
Does it have any purpose?
The process of folding and recoiling leaves costs the Mimosa pudica a lot of energy (Source: https://link.springer.com/article/10.1007/BF00196975). While the leaves are bent, the opportunities to photosynthesize and produce energy are also reduced. If it causes the plant to waste energy and lose opportunities to generate more energy, why would any plant have such a wasteful mechanism?
Researchers believe that the trait evolved as a way to avoid predators. If a herbivore comes along and begins to munch on the Mimosa plant, bending the leaves inwards and drooping would make it harder for the predator to tear the leaves off, and it would also serve to expose thorns. Drooping also gives the impression that the plant is dead or dying, and therefore not as appealing as fresher, potentially more nutritious plants. Furthermore, it’s also believed that such rapid movements help the plant dislodge insects that may pose a danger to certain parts of the plant.
There is always an energy trade-off when it comes to such predatory avoidance behaviors and Mimosa plants do take a risk every time they close. Studies on the effect of the intensity of light have shown that the Mimosa plant takes more time to reopen its leave under high light conditions, which means it is willing to take a greater risk to avoid predators when light is abundant.
However, Mimosa’s response isn’t consistent across all stimuli. Just as animal responses become dulled or get habituated to certain repeated stimuli, especially those that aren’t life-threatening (like a loud repeating sound), research has shown that the Mimosa plant’s movements also show habituation. A study published in 2014 showed that under favorable environmental conditions, Mimosa pudica shows learning behaviors towards certain stimuli. Plant behavioral research is still in its nascent stages, but it holds exciting potential to teach us about how life “learns” and how such behaviors might have evolved.