Open stoma located on the lower surface of an orchid leaf. In contrast to other parasitic plants, the majority of orchids have retained their ability to photosynthesize. Photosynthesis is a light dependent process which uses carbon dioxide and water to make energy for the plant whilst releasing oxygen as a waste gas. One open stoma (pore) flanked by two guard cells is visible in the upper left corner of the image. Guard cells control the opening and closing of stomata as required to allow exchange of gases into and out of the leaf. Scanning electron micrograph; x 600; stoma is approximately 20 micrometers wide. Supplied by Lauren Holden, courtesy of Wellcome Images.