The transpiration of Helxine solierolii ("Mind your own business") was measured by growing it in soil kept close to field capacity on the pan of a weighing machine in a glasshouse in southern England. The glasshouse was not heated except to exclude frost. The plants provided a nearly horizontal mat of green foliage at all times of the year and the monthly mean daily transpiration, E', was related to the corresponding solar radiation R'c falling on the plant surface and to the vapour-pressure deficit ea-~a of the outside air by the following equation: E' -0.38 R'c + 0.17 (ca-ca)-0.17 mm/day where R'c is expressed as evaporating power in mm of water, e,, is the saturation vapour pressure (mm Hg) at the mean of the daily maximum and minimum air temperatures, and ed the actual vapour pressure (mm Hg) of the air, determined from the wet, and dry-bulb temperatures at 09h00 daily.
R'c can be estimated from the solar radiation measured outside, if the light transmission of the glasshouse is known, so that the data required for estimating E' can all be obtained from agro-meteorological stations and the equation promises to be of practical value in calculating the water requirements of commercial glasshouse crops more accurately than has been hitherto possible.