Satisfaction of a leaf's need for CO<latex>$_2$</latex> 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<latex>$_2$</latex> concentration in the guard cells triggers (a) uptake of K<latex>$^+$</latex> in exchange of H<latex>$^+$</latex>, (b) production of organic acids, and (c) import of Cl<latex>$^-$</latex>. '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<latex>$_2$</latex>, 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<latex>$_2$</latex> 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<latex>$_2$</latex> assimilation or to water husbandry.