Why Do Touch-Me-Not (Mimosa Pudica) Leaves Close When Touched?

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 plantt or the sensitive plant, 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 it a lot as a kid, and still do, whenever a chance presents itself.

There were a lot of those ‘kid stories’ that surrounded this fascinating behavior of the plant’s leaves (some of which I believed, by the way). As we grew older, the stories transformed a bit to accommodate the real biological reason behind the great touch-me-not mystery.

So, without further adieu, let’s look at why touch-me-not leaves “go to sleep” when you touch, shake or blow air at them.

What is Mimosa Pudica (Touch Me Not plant)?

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, is well-known for closing its leaves (or folding its leaves inwards) when touched.

mimosa pudica

The famous Touch-Me-Not plant (Image Source: Wikimedia)

There are certain plant species that undergo structural changes in a very short span of time; this is known as ‘rapid plant movement‘. This happens when the receptors present in the plant’s body are activated by an alteration or modification of the plant’s shape. Mimosa pudica, better known as the ‘touch-me-not plant’, is one of the few plants that demonstrate this rapid plant movement. Although native to South and Central America, touch-me-nots are now found all over the world due to their amazing response to human touch.

Why do mimosa pudica leaves close when touched?

The movement of plants caused by touch stimulus is known as Thigmonasty. In this mechanosensory response, water within the cells and other cell contents apply a certain amount of force against the cell walls of the plant; this is called turgor pressure.

It is due to turgor pressure that the leaves of this plant stay upright unless disturbed externally. Now, when you touch or shake the leaves (known as seismonastic movements), the swollen base of the leaf stalk (called the ‘pulvinus’), which contains certain contractile proteins, is activated.

The mechanism that makes mimosa pudica leaves to close

When disturbed externally, certain regions of the plant trigger a release of various chemicals, including potassium ions, within the body of the plant. These chemicals make water and electrolytes flow/diffuse out of the cell, resulting in a loss of cell pressure. This causes the cell to collapse, which squeezes the leaves shut. Stimuli, in the form of touch, is sometimes transmitted to neighboring leaves as well, causing something like this to happen:

via GIPHY

Does it have any purpose?

The process of folding and recoiling of leaves costs the mimosa pudica plant a lot of energy. Furthermore, this response is also said to interfere with photosynthesis (the process through which leaves prepare food for the plant in the presence of water, carbon dioxide and sunlight). Therefore, it comes as no surprise that botanists are confounded as to why these plants evolved this trait, given that it depletes their energy stores and adversely impacts them.

There is no conclusive proof as to why touch-me-not plants evolved this trait, but researchers believe that it may act as a defense mechanism of sorts for them. Consider this; if you were a tiny little herbivorous insect looking for a nice supper from a plant, would you take the risk and go near a plant whose leaves twist, turn, fold and droop when you touch it, or would you rather go to a passive, peaceful meal?

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.

As enthralling as it may seem at first, touch-me-not leaves have to work extra hard to make their food after you touch, shake or otherwise disturb them in any way. Therefore, as the sensible and noble human being that you are, try not to have fun at its ‘energy expense’ the next time you stumble upon a touch-me-not plant.

It might be a fascinating process to watch, but just because you can doesn’t mean you should!

References

  1. National Gardening Association
  2. Plant Physiology – Journal
  3. National Academy of Sciences
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About the Author:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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