Stoma on a rose leaf. Coloured scanning electron micrograph (SEM) of the lower leaf surface of a Garden Rose Rosa sp., showing an open stoma. Epidermal cells (green) occur around this structure. A stoma (plural: stomata) is a tiny pore bordered by two kidney-shaped guard cells. Here the partly open pore is elongated in shape. Guard cells serve to open and close the stoma: opening the pore allows gases to be exchanged by leaf tissues during photosynthesis; and closing the pore at night or during dry periods prevents water loss. In the rose, more stomata occur on the underside of leaves. Magnification: x850 at 5x7cm size. Magnification: x2,500 at 6x4.5inch size.