Leaf stomata. Light micrograph of a vertical section down through the surface of a leaf from a Scots pine tree (Pinus sylvestris), showing a stoma (central space) surrounded by two guard cells (left and right). The surface of the leaf is across top. The stoma, an opening in the surface of a leaf, is opened and closed as the guard cells expand and contract. This regulates the passage of air into and out of the leaf through the stoma. The surface of a plant is covered with millions of stomata.