English: Water filtration in the roots of the mangrove Rhizophora stylosa

(a) Schematic of an RS root. The outermost layer is composed of three layers. The root is immersed in NaCl solution.

(b) Water pass through the outermost layer when a negative suction pressure is applied across the outermost layer. The Donnan potential effect repels Cl− ions from the first sublayer of the outermost layer. Na+ ions attach to the first layer to satisfy the electro-neutrality requirement and salt retention eventually occurs.