This causes them to close their stomate in order to prevent
transpiration.
CAM plants keep their stomate closed during the day and opens
them in the night. This controls the amount of water lost b/c water
won't leave the plant as much when it's dark outside. The setback
to CAM plants is that they take forever to grow this way
(cactii).
C4 plants open their stomate during the day at different
irregular times. This helps the plant from experiencing plasmolysis
and dying to the extremely hot weather.