Guard cells are bean-shaped cells and are found in pairs, creating a mouth-shaped epidermal opening called stoma (plural stomata). These cells are surrounded by epidermal cells proper. Unlike the other epidermal cells proper, guard cells contain chloroplasts, thus photosynthetically active. Stomata mainly occur in the epidermis of the leaves, but sometimes they are found on the other parts of the plants such as stems or fruits. A stoma makes the passage of gas exchange between the plant tissues and the environment. In addition, it allows the water vapor diffusion. Guard cells control the rate of gas exchange and water diffusion by changing the size of the stomata.