Xerophytes close their stomata during the day in order to conserve water. Closed stomata reduces transpiration and hence reduce water loss. Their stomata are open during the night, and it is then that they take in carbon dioxide. They store it as malate and other, simple organic compounds. Crassulacean acid metabolism allows plants to close their stomata during the day in order to conserve water and carry out photosynthesis with CO2 that is taken up during the cool, humid night.
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