This is a stoma. Stomata (plural) are microscopic pores on plants that allow gases like oxygen and carbon dioxide to pass in and out of the plant. The stoma also allows water to escape into the atmosphere. When the stoma opens it allows air (containing the oxygen it needs) in, and oxygen, gas, and water out. Plants must be careful to control this so they get the oxygen they need, but don’t let too much water escape. It has been found that the number of stomata increases in areas where rainfall is abundant and decreases where rainfall is lower.