A hydathode is a type of secretory tissue in leaves, usually found in Angiosperms, that secretes water through pores in the epidermis or margin of leaves, typically at the tip of a marginal tooth or serration. Hydathodes are mainly found in aquatic plants and in some herbaceous plants growing in moist places. They probably evolved from modified stomata, and are connected to the plant vascular system by a vascular bundle. Hydathodes are commonly seen in water lettuce, water hyacinth, rose, balsam, and many other species.
Hydathodes are made of a group of living cells with numerous intercellular spaces filled with water, but few or no chloroplasts, and represent modified bundle-ends. These cells (called epithem cells) open out into one or more sub-epidermal chambers. These, in turn, communicate with the exterior through an open water stoma or open pore. The water stoma structurally resembles an ordinary stoma, but is usually larger and has lost the power of movement.
Hydathodes are involved in the process of guttation, in which positive xylem pressure (due to root pressure) causes liquid to exude from the pores. Some halophytes possess glandular trichomes that actively secrete salt in order to reduce the concentration of cytotoxic inorganic ions in their cytoplasm; this may lead to the formation of a white powdery substance on the surface of the leaf.
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