A leaf must let in carbon dioxide (CO₂) to make sugar, but the cells that take it up are wet. A wet surface open to the air loses water. The leaf solves the problem with stomata (singular: stoma), tiny pores in the leaf surface, often mostly on the underside. A typical leaf has from dozens to several hundred of them per square millimetre.

Each stoma is bordered by two guard cells. When the guard cells take up ions and water by osmosis, they swell and bend apart and the pore opens. When they lose water, they go slack and the pore closes. Typical signals:

• Light (especially blue light) opens stomata, so that CO₂ is available while photosynthesis runs.
• Low CO₂ inside the leaf opens them.
• Water stress makes the roots and leaves release the hormone abscisic acid (ABA), which makes guard cells lose water and the pore close.