pulvinus

Pulvinus is a swollen, joint-like region at the base of a leaf or leaflet that functions as a hinge for movement. In many flowering plants, pulvini occur at the junction between the petiole and the leaf blade or at the base of individual leaflets in compound leaves. They enable rapid, reversible leaf movements by adjusting turgor pressure within specialized parenchyma cells.

Anatomy and position: The pulvinus consists of two opposing groups of motor cells surrounded by epidermis and connected to the leaf’s vascular system. These cells have large central vacuoles and are capable of quick changes in volume, allowing the pulvinus to bend the leaf or leaflet without growth.

Mechanism: Movement arises from changes in turgor pressure driven by osmotic shifts. Active transport of ions such as potassium and chloride creates osmotic gradients that draw water in or out of the motor cells via aquaporins. Water efflux on one side causes the pulvinus to droop, while water influx or reabsorption restores turgor and returns the leaf to its extended position. Movements can be rapid, as in seismonastic responses to touch, or slower and daily, as in nyctinastic movements.

Function and examples: Pulvinus-driven movements help protect leaves from damage, optimize light interception, or regulate temperature and water use. The best-known example is Mimosa pudica, whose leaflets fold in response to touch. Other species within Fabaceae and related groups also possess pulvini that contribute to leaf or leaflet folding and unfolding.