Plant stoma, coloured environmental scanning electron micrograph (ESEM). A stoma is a pore that regulates the exchange of gases and water vapour into and out of the plant. The highest concentration of stomata is found on the leaves of a plant, but they are also found elsewhere. The opening and closing of the stoma is controlled by semicircular guard cells (pink), which swell and become turgid to open the stoma, closing it when relaxed. This stoma is from the underside of a leaf from a chameleon plant (Houttuynia cordata).