Stomatal density refers to the number of stomata present on plant leaves per square millimeter. Leaf stomata are small pores that facilitate gas exchange in plants. These pores allow carbon dioxide to be absorbed and water to be released.
Stomatal density can vary depending on the environmental conditions of the plant. High stomatal density allows a rapid intake of carbon dioxide; however, depending on the environment of the plant, this may be dangerous due to a higher water loss. Conditions that lead to a higher stomatal density include low carbon dioxide content, high humidity and plenty of sunlight. Research into the stomatal density of fossilized plants also provides insight into the atmospheric conditions on Earth during the time that the plant lived.