CHEMICALS which close stomata have been described by Zelitch1. When sprayed on leaf surfaces, the compounds decrease transpiration and photosynthesis of detached leaves2 and of maize plants in soil3 by increasing the diffusion resistance of stomata. When sprayed on intact tobacco plants in the greenhouse2 or on sunflowers growing in bins of soil outdoors3, the compounds reduced the loss of water from the soil. It remained to demonstrate the effect of chemical closure of stomata on transpiration by a population of plants in the field.