Nature has evolved all strategies to combat heat and drought. Plants which look like stones store carbondioxide during night time in their cell vacuole in the form of malate derived from dark form of Phosphoenolpyruvate carboxylase which fixed carbondioxide during dark in cytoplasm.
During day time this malate is transported to cytoplasm again from the vacuole where it is decarboxylated and the carbondioxide thus released is fixed again in regular fashion.
This is done to keep stomata closed during day time when temperatures are high and it could harm the plants to transpire too much . However malate has now been found to have manyfold functions in plants.