Which one of the following plants keeps its stomata open during the night and…

Most ordinary land plants (C3 plants) open their stomata during the day to take in CO2 for photosynthesis and close them at night. Plants adapted to hot, arid habitats face heavy water loss if they keep stomata open in daytime heat, so several succulents follow Crassulacean Acid Metabolism (CAM): they keep stomata shut through the day to conserve water, and open them at night when temperatures are lower and humidity higher, fixing atmospheric CO2 into organic acids (such as malic acid) stored in the vacuole; during the following day, with stomata closed, the stored acid is broken down and the released CO2 enters the Calvin cycle (the light-independent carbon-fixation reactions), using the ATP and NADPH generated by the daytime light reactions.

Among the given options, Cactus is the classic desert succulent that follows this CAM adaptation: its stomata remain shut through the hot day, minimising transpiration loss, and open at night to admit CO2.

Note: this exam's official answer key treats Cactus as the intended example of this adaptation, even though a minority of orchid species are also documented as CAM plants (see the Orchid option's own explanation).

• Water-lily: a floating-leaved hydrophyte whose stomata, concentrated on the upper leaf surface, open during the day for ordinary C3 photosynthetic gas exchange.

• Cycas: a gymnosperm with a cone-bearing, palm-like habit that carries out ordinary C3 photosynthesis, opening stomata during the day and closing them at night.

• Orchid: an extremely diverse family in which a minority of species (mostly seasonally-dry epiphytes) use CAM, while most orchids carry out ordinary C3 photosynthesis with daytime stomatal opening.