Mimosa, the shy plant, how they close?

This morning, when I was browsing Steemit, I saw @papa-pepper post about "Mimosa Pudica -one shy plant" post and decided to write this to explain the mechanism behind how this shy little plant close their leaves upon physical stimuli.

Usually plants moves by growth, like growing towards the sun, or coiling around the fence (like that of grapes and tomato), those "slow movements" mediated by growth are generally known as tropism. But some plants evolved to exhibit rapid plant movements like the mimosa or the venus fly traps.

a mimosa that i found in my university

One of the mechanism that a plant employs for rapid response is through the movement by water, or more specific osmosis
Osmosis refers to the movement of water across a membrane according to the water potential. In simple words, where there are more of salts (sodium, potassium, chlorides etc) and sugar (glucose, fructose, sucrose), the water will tends to move there.

So in the case of the mimosa, upon a stimulus is detected by the plant, a cascade of electric signal will reach the pulvinus, the base of the leaflets connecting the stem. This will lead to the outflowing of potassium ions to leave the cells, leading to less water within the cell. With less water, the cell is like a balloon with less air, it becomes deflated and flaccid. So when the upper part of the pulvinus is flaccid, while the bottom part of the pulvinus are more turgid.

The turgidity difference between the upper and lower part of the cells in pulvinus leads to closing of the leaflets. You can also get two long ballons and stick them together and blow them up with different volume, they will bend towards the less turgid side.

The use of water movements in responding to environmental stimuli in plant is actually quite common. just that they are in microscopic level, and weren't as obvious as that tissue movements in mimosa.

Many leafy plants contain tiny openings for gaseous exchange or regulate water lost, they are call the stoma. The opening and closing of the stoma is also regulated by the water movements into and out of the guard cells.

Picture from wikipedia

Here I will end it with a famous quote from our Hong Kong star Bruce Lee, be formless, shapeless like water, if you put water into a cup it becomes the cup.

Nature has lot of things to amaze us with, so be kind and friendly to our environments, a lot of things await us for discovery~

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