The osmotic values in the leaves of Thalassia testudinumBanks ex Konig, collected from the natural habitat, were found to be 32 atm; the relative transpiration was 100%. Electronmicroscopical analysis of the cuticle reveals many perforations. The critical sublethal water deficit lies near 65%. In dense meadows the green surface area of the plants amounts to 18.56 m2/m2 bottom area.