Openings in the epidermis of leaves which are controlled by special cells.

Photosynthesis of chloroplasts consumes carbon dioxide and produces oxygen. Because of the cuticle, these gases can only be exchanged between the internal air space of the leaf and the external air when stomata are opened. This exchange, however, results in the loss of water by the plant. Therefore, depending on external conditions, plants have to regulate their stomata in order to minimize water loss and maximize photosynthesis.