Rhizomes of Hydrocotyle plants from three contrasting habitats were cloned and the ramets grown under controlled environmental conditions. Measurements of net photosynthesis, transpiration, and total leaf diffusion resistance were used to examine possible physiological adaptations to specific field environments. Increasing dryness of the growth chamber environment had large effects on gas exchange (CO2 and water vapor) and on total diffusion resistance of plants from a pond, moderate effects on plants from a mesic forest, but plants from a coastal sand dune were unaffected by the experimentally imposed dryness. Thus the 3 Hydrocotyle types demonstrated adaptive physiological reponses to their specific field habitats. Periodic stomatal oscillations were induced in ramets from the pond by sharply increasing irradiance, but the adaptiveness of the oscillations cannot be determined with the evidence at hand. No stomatal closure could be induced by atmospheric dryness alone as long as soil and plant dessication were prevented. There were no observable differences in stomatal response to increasing atmospheric vapor pressure deficits.