Cabbage leaf stomata. Coloured scanning electron micrograph (SEM) of stomata (pores, oval) on the surface of a leaf from a cabbage (Brassica sp.) plant. Stomata are pores which regulate gas and water exchange between the leaf and the atmosphere. During the day they open (as seen here) to allow the exchange of gases during photosynthesis, closing at night to prevent water loss. They are opened and closed by two bordering guard cells. Magnification: x470, when printed 10 centimetres wide.