Gas exchange between a leaf and the atmosphere occurs through nonselective, adjustable stomata in the epidermis. A stoma is defined by the surrounding pair of guard cells, which are kidney-shaped epidermal cells that face each other. Certain conditions stimulate the guard cells to accumulate potassium salts. Constraints imposed by the cell-wall architecture force the cells to bow outward upon the consequent osmotic-water influx; this deformation enlarges the pore. Stomal closure occurs when the guard-cell pair loses solutes. Thus, from moment to moment, the aperture size is a compromise between the opposing priorities of permitting CO₂ uptake and avoiding H₂O-vapor loss. As CO₂ is required for photosynthesis and water is usually the most limiting resource for a terrestrial plant, regulation of the pore size is perhaps the most crucial aspect of a plant's physiology.

Guard cells have evolved special attributes to fulfill their critical role. They are small and have enhanced capacity for ion flux. Their chloroplasts are specialized for starch storage and mobilization. Almost uniquely, they lack protoplasmic connections With adjacent cells, and their vacuole is a dynamic structure. As these cells have complex ion-flux patterns and respond to a variety of stimuli, they have become the model system for plant electrophysiology and signal transduction. The unusual pattern of primary carbon metabolism has long intrigued plant biochemists, as their differentiation has intrigued plant developmental biologists; more recently, guard cells have attracted molecular biologists. The implications of guard-cell function to the utilization of finite resources such as water and soil and to agricultural productivity justify our intense efforts to understand them better.