Evapotranspiration rates increased with leaf area and fell with its decline at the time of flowering of cotton plants ( L.) and this relation was maintained while the rate of pan evaporation was steadily decreasing. A similar relationship was established for the hyacinth bean ( L.) but evapotranspiration dropped at the onset of flowering, at the time when leaf area was still increasing and the evaporating potential of the atmosphere began to rise. Evapotranspiration per unit area of leaf surface per day declined progressively with age for both species.