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They just want to protect themselves. The closing of the leaves is a natural defense in reaction to any kind of touch, perceived by the plant as a potential attack. “If a predator touches these leaves, he will move away when they close”, says biologist Sônia Perez, from the Federal University of São Carlos (UFSCAR). In the plant known as sensitive or sleepyhead (Mimosa pudica), the response is extremely agile. At the base of each leaflet, there are cells capable of losing water very quickly. This happens when the plant receives an external stimulus – touch, for example. In this case, two chemicals present in your body – potassium and calcium – direct water into a space between cells. This causes them to wither, shrinking the leaves. “If each leaf were an arm, these base cells would play the role of the elbow, which changes the leaf’s position according to the stimulus”, says Sônia.
The curious thing is that the stronger the touch, the greater the number of leaves that close. But the effect is temporary: after a while without being touched, the plant restores the water balance inside and the leaves open again.