Guard cell

Each stoma is surrounded by a pair of guard cells. These cells change shape as they gain or lose water, opening or closing the pore. An open pore allows gas exchange, but also allows water vapour to escape.

Each stoma is a pore in the leaf surface surrounded by a pair of guard cells. When water enters the guard cells by osmosis, they become turgid. Their unevenly thickened walls cause them to curve apart, opening the pore.