Which side of a leaf takes in gases from the air? Leaves have tiny openings called stomata (small pores that let gases move in and out). You can block these openings to find out where they are. Choose two types of plants. Coat the top of four leaves on each plant with a thick layer of Vaseline. Coat the underside of four other leaves on each plant the same way. Observe all the leaves every day for one week. Leaves coated on the underside are more likely to die. That shows the stomata are mostly on the bottom of the leaf. Without those openings, the leaf cannot take in carbon dioxide or release oxygen.