hydathodes

Hydathodes are specialized structures in the epidermis of some plants that form tiny pore-like openings at leaf margins or at the termini of veins. Through these openings, xylem sap can be exuded as water droplets, a phenomenon known as guttation.

Anatomy and location: Hydathodes typically consist of a cluster of epidermal cells surrounding a small pore. The cuticle over the pore is thin or absent, allowing liquid to move from the leaf interior to the surface. In many species the pores are bordered by a ring of elongated epidermal cells, and the openings are often located at the leaf edge or at vein endings.

Mechanism and conditions: Guttation occurs when root pressure increases and drives water and dissolved minerals through the xylem to the leaves, especially during periods of low transpiration such as cool, humid nights. High soil moisture, low transpiration, and moderate temperatures promote droplet formation at hydathodes, whereas drought or dry air reduces guttation.

Distribution and ecological notes: Hydathodes are common in many herbaceous dicots and in some monocots, including certain grasses, but they are not universal among plants. The droplets released by hydathodes contain water plus dissolved minerals and organic compounds transported from the plant. The exact ecological role of guttation is not fully settled; it is thought to help relieve excess internal water pressure and solute accumulation, and may contribute to salt regulation in leaves.

See also: guttation.