Antitranspirants conserve water and maintain favorable plant water balances by reducing stomatal apertures, by forming a thin film over the leaves, or by reflecting excessive radiation. Under normal conditions, reductions in both transpiration and photosythesis are to be expected, but reduction in growth does not always occur, and need not always be disadvantageous when it does. Antitranspirant effects on stomata were judged indirectly by porometry (mass flow or rate hygrometer), and directly by microscopic measurement of stomatal apertures from epidermal peels. The relationship between diffusive resistance and stomatal aperture was determined. It was found that a film antitranspirant, by increasing plant water potential, actually increases the aperture of stomata lying immediately under it. Direct stomatal measurements enabled assessment of the effects of the stomatal-closing antitranspirant, phenylmercuric acetate (PMA). It was discovered that PMA not only retards stomatal opening, but also retards closure, thereby increasing transpiration under conditions of low light or low water supply. (Author)