The leaf’s hinge is a water-powered joint
A sensitive plant’s leaf does not bend because it has a muscle. At the base of each leaflet is a small swelling called a pulvinus, a hinge-like structure packed with water-filled cells. Their internal pressure keeps the leaflet extended, much as air pressure helps a flexible structure hold its shape.
When the plant is touched, an electrical and chemical signal travels through the leaf. Cells on one side of the pulvinus move charged particles out, and water follows. As those cells lose water, their pressure drops; the change in pressure makes the leaflet fold. Neighboring joints can respond in sequence, so a touch in one spot may ripple through a larger part of the leaf.
Why the leaves rise again
The collapse is reversible. After the signal subsides, the cells take up ions and water returns, restoring their pressure. The leaflets gradually open, though recovery can take longer than the initial fold. The plant is changing the shape of a living hinge, not contracting and relaxing tissue like an animal muscle.
Folding may make the plant less attractive or harder to eat for some herbivores, but the response also costs the plant resources and can be triggered by harmless contact. Its quick movement is therefore a striking example of a tradeoff: a plant can react dramatically without muscles, using shifts in water pressure to move its own leaves.
