A stoma is a pore located on the lower or upper layer of the epidermis of a plant leaf through which water evaporates, and gas exchange occurs. The stoma opening can expand and contract, regulating transpiration and gas exchange. Under the conditions of limited water supply, the guard, and the stoma gap is closed. When there is a lot of water in the closing cells, it presses against the walls, and thinner walls stretch more strongly, while the thicker ones draw inward, a gap appears between the closing cells. An airy cavity is located below the stoma and is surrounded by leaf pulp cells, through which gas exchange occurs. Air containing carbon dioxide and oxygen penetrates the leaf tissue through these pores and is then used in the process of photosynthesis and respiration. At the entrance, the air mixes with water vapor evaporating from the leaf, and therefore the plant cannot get carbon dioxide without simultaneously losing some water. Many plants have protection against evaporation of water in the form of wax-clogging stoma.