Mimosa pudica, the Bashful herb

The most notable feature about this plant is the manner in which the leaves fold inwards when touched. The image is not a time-lapse; the movement is easy to observe in real-time, and actually happens faster than the speed shown in the animated gif.

How does it happen? Through a process known as ‘thigmonasty’, similar to how a Venus Flytrap can trap insects, it all comes down to action potentials and tugor pressure. An action potential happens when the electrical membrane potential of a cell rises and falls rapidly; this happens in certain types of cells including neurons, muscle cells and some plant cells. In Mimosa pudica, the leaves fold because of the internal movement of water. The external stimulus (wind or touch) triggers certain areas of the stem to expel potassium and chlorine ions, and absorb calcium ions. This causes an osmotic gradient that draws water out of the affected leaves. The lower cells of the leaf temporarily shrink due to the loss of water, and it can no longer support the leaf; the entire leaf curves downwards like a fan, and droops. It is also thought that vacuoles in the plant cells provide temporary high-speed storage for the calcium ions.

Why does it happen? It is thought that M. pudica evolved this curious ‘avoidance’ behavior as a defense against predators. Of course plants need light to photosynthesize, and therefore closed leaves are a negative trade-off for the plant. Indeed, scientists have found that by manipulating the light environment of individual M. pudica plants, they can predict how long the leaves stay closed. So for example the leaves stayed closed for longer under high light conditions, and the leaves re-opened faster under low light conditions. This suggests that the plant balances the risks and the rewards of this behavior and decides accordingly whether being ‘shy’ is worth skipping a meal for 🙂

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