IntroductionIt has recently been emphasized (14,15, 19, 20,32) that the two main environmental conditions that influence the development and health of ordinary leafy plants, as far as water relations are concerned, are (a) the capacity of the soil to deliver water to the plant roots and (b) the capacity of the aerial surroundings to permit water loss from the foliage through transpiration. The second of these conditions may be measured and recorded in a fairly satisfactory way, by means of atmometers of various forms, and numerous studies on the relations between evaporation and plant