Under favorable light and temperature conditions the possibility and rate of photosynthetic primary production is determined by both water and C02availability at the sites of carboxylation. The uptake of CO, from a dry atmosphere into a well-hydrated assimilatory tissue is unavoidably accompanied by loss of water. In arid regions a continued shortage of water combined with a high atmospheric demand is correlated with low photosyntheric capacity and thus low plant productivity. Under the extreme climatic conditions encountered in desert regions, selection pressures have favored the evolution of specialists such as plants exhibiting C4and CAM photo synthetic pathways. These homoiohydric organisms are able to maintain metabolic activity over long, stressful dry periods. In contrast, photosynthesis in poikilohydric lichens at the desert site, is restricted to short periods when the thallus is moistened either by rain, fog, high air humidity, or by dew condensation. After a few hours or after even shorter periods of active metabolism, the lichen dries out and becomes inactivated. The efficiency and rate of water uptake, the length of time of water re tention and the effectiveness with which the favorable hydration period is used for photosynthetic CO, uptake are decisive factors in determining the productivity of a lichen in a desert habitat. For Ramalina maciformis (Del.) Bory, a species common in the Negev Desert, Israel, the relationships between water status and net photo synthesis have been investigated from an ecophysiological point of view.