Drought tolerant plants/Drought resisting xerophytes make use of either C4 carbon fixation or crassulacean acid metabolism (CAM) to fix carbon during photosynthesis. CAM is particularly good for arid conditions because carbon dioxide can be taken up at night, allowing the stomata to stay closed during the heat of day and thus reducing water loss. The CO2 is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.