CAM plants have a process similar to C4 plants, but they only open their stomata at night as the live in very extreme heat (mostly deserts). They take in CO2 and incorporate it into a 4-carbon sugar, which they then store. Then, they retrieve the sugar and begin the Calvin cycle during the day. This allows the plant to keep its stomata closed during the extremely hot day and prevent dehydration. The cactus in this photo is a CAM plant.