1. Light generally causes stomata to open and darkness to close. In some succulent plants however, this trend is reversed and the plants open their stomata at night when they can take advantage of the cooler evening temperatures for gaseous exchange, thus reducing water loss.
2. Wilting or water deficit causes the stomata of a plant to close thus preventing further water loss from the leaves.
3. An increase in CO2 causes stomatal closure.
4. Increase in temperature causes stomata to open.
A hydathode is a type of secretary tissue in leaves, usually of Angiosperms, that secretes water through pores in the epidermis or margin of leaves, typically at the tip of a marginal tooth or serration. They probably evolved from modified stomata. It is involved in guttation, where water is released from the top in order to transport the nutrients in the water from the roots to the leaves. Hydathodes are connected to the plant vascular system by a vascular bundle.
Since the liquid being extruded is from the xylem it also contains salts, sugars, and organic compounds dissolved in water and this is sometimes seen to crystallize on evaporation, forming a white powdery substance on the leafs edge. This crystallisation is very obvious in halophytes, plants adapted to live in high salt environments, and consequently the hydathodes are known as salt glands in those species.
Hydathodes are special structures through which exudation of water takes place in liquid form.They are mainly found in aquatic plants and in some herbaceous plants growing in moist places. They occur at the apices of the veins at the tips of the leaves or on their margins. Hydathodes are made of a group of living cells with numerous intercellular spaces filled with water, but few or no chloroplasts.
They 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 stomata or open pore. The water stoma structurally resembles an ordinary stoma, but is usually larger and has lost the power of movement. Hydathodes are commonly seen in water lettuce, water hyacinth, rose, balsam, and many others.