Moving air into and out of the body – breathing – is required for more than respiration. For photosynthesis, land plants breathe in carbon dioxide (CO2) to make food (using energy from light) and breathe out oxygen.
This gas exchange occurs via microscopic pores called stomata: mouth-like structures formed from two guard cells that resemble a pair of lips, which can close to prevent moisture loss.
If an aquatic plant were covered in tiny holes, however, it would drown. So in water lilies and other flowering plants with floating leaves, only the upper side of a leaf has stomata; those pores are permanently open because aquatic plants don’t need to worry about losing moisture.
Some flowering plants are adapted to a submerged lifestyle: seagrasses lack stomata and carry out photosynthesis, but must rely on bacteria in ocean sediments to provide a source of carbon in the absence of CO2.
