A composition is disclosed for application to a plant that comprises an exogenous chemical (for example, a postemergent herbicide), an aqueous diluent, and a first excipient substance that is amphiphilic. The weight/weight ratio of the first excipient substance to the exogenous chemical is between about 1:3 and about 1:100. The aqueous composition forms anisotropic aggregates on a wax layer, and the presence of the anisotropic aggregates can be detected by a test described herein. Compositions of the present invention, when applied to plants, provide enhanced biological activity per unit amount of exogenous chemical, as compared to otherwise similar compositions containing surfactants that do not form anisotropic aggregates. Without being bound by theory, it is presently believed that this enhanced biological activity results from the formation or enlargement of hydrophilic channels through the epicuticular wax of the plant.