Certain plants, especially succulents, which grow under semi-arid conditions, fix atmospheric CO2 in dark. The process was first observed in plants belonging to family Crassulaceae and termed as crassulacean acid metabolism. In CAM plants, stomata are open at night and remain closed during most of the day. The diurnal fluctuation in acidity in CAM plants is due to change in amount of vascular malic acid in mesophyll cells. During night, malic acid is synthesized utilising, CO2 which then accumulates in the vacuole and may account for about 85% of the total titrable acid content. During day this malic acid is consumed resulting in the decrease of acidity.