Abstract
Recent reviews have denied the applicability of the classical theory of stomatal movement. The newer explanations are shown to be incorrect, and the major objections to the classical theory invalid. Nevertheless, the classical theory needs to be modified. If the decisive factor is assumed to be carboxylic acid (RCOOH) rather than CO2 concentration, all the known facts can be explained. Two predictions of this modified classical theory were vindicated. The proposed relationship of stomatal opening to RCOOH concentration is illustrated schematically.