Calcium oxalate crystals are widespread among animals and plants. In land plants, crystals often reach high amounts - up to 80% of dry biomass. They are formed within specific cells, and their accumulation constitutes a normal activity rather than a pathological symptom, as occurs in animals. Despite their ubiquity, our knowledge on the formation and the possible role(s) of these crystals remains limited. We show that the mesophyll crystals of Amaranthus hybridus (pigweed) exhibit diurnal volume changes with a gradual decrease during daytime and a total recovery during the night. Moreover, stable carbon isotope composition indicated that crystals are of non-atmospheric origin. Stomatal closure (under drought conditions or exogenous application of abscisic acid) was accompanied by crystal decomposition and by increased activity of oxalate oxidase that converts oxalate into CO2. Similar results were also observed under drought stress in Dianthus chinensis, Pelargonium peltatum and Portulacaria afra. Moreover, in pigweed, despite closed stomata, the leaf metabolic profiles combined with chlorophyll fluorescence measurements indicated active photosynthetic metabolism. In combination, calcium oxalate crystals in leaves can act as a biochemical reservoir that collects non-atmospheric carbon, mainly during the night. During the day, crystal degradation provides subsidiary carbon for photosynthetic assimilation, especially under drought conditions. This new photosynthetic path, with the suggested name "alarm photosynthesis", seems to provide a number of adaptive advantages such as water economy, limitation of carbon losses to the atmosphere and a lower risk of photoinhibition, roles that justify its vast presence in plants.