Microscope image of the epidermis of a spiderwort leaf with well-organized stomata in high density. Stomata (singular stoma) are like little mouths on the leaf surface, specialized in gas-exchange – CO2 enters a plant through them. The opening and closure of stomata are tightly controlled, because when stomata are open, water is escaping from the plant too. Therefore, each plant needs to find a delicate balance between gas uptake and water loss. Density of stomata can vary due to environmental factors such as light, humidity, and CO2 concentration. In fact, there are studies using density of stomata preserved on surfaces of fossil leaves to predict how the Earth’s atmosphere was like millions of years ago.