Satisfaction of a leaf’s need for CO 2 requires an intensive gas exchange between mesophyll and atmosphere; prevention of excessive water loss demands that gas exchange be kept low. Stomata open when a low CO 2 concentration in the guard cells triggers ( a ) uptake of K + in exchange of H + , ( b ) production of organic acids, and ( c ) import of Cl - . ‘Hydropassive’ stomatal closure (i.e. turgor loss without reduction of the solute content of the guard cell) appears insufficient to protect the plant from desiccation. An additional ‘hydroactive’ solute loss is necessary; it is brought about by (+)-abscisic acid (ABA) acting as feedback messenger between mesophyll and epidermis. Stomatal closure not only curbs water loss but improves water-use efficiency because transpiration is proportional to stomatal conductance (at constant temperature). In contrast, assimilation, following saturation kinetics with respect to intercellular CO 2 , is relatively insensitive to changes in stomatal conductance (as long as stomata are wide open). In Xanthium strumarium , the amplitude of stomatal responses to ABA depends on the concentration of CO 2 in the guard cells; the opposite statement is also true. These interactions cause stomata to behave like ‘adjustable control systems’ capable of giving priority either to CO 2 assimilation or to water husbandry.