In this Experiment, students make impressions of leaf stomata (using clear nail polish) and test a hypothesis of their choice about how leaf stomata density might vary under different environmental conditions. Leaf stomata are the principal means of gas exchange in vascular plants. Stomata are small pores, typically on the undersides of leaves, that are opened or closed under the control of a pair of banana-shaped cells called guard cells. When open, stomata allow CO2 to enter the leaf for synthesis of glucose, and also allow for water, H2O, and free oxygen, O2, to escape. In addition to opening and closing the stomata (stomata behavior), plants may exert control over their gas exchange rates by varying stomata density in new leaves when they are produced (such as in the spring or summer). The more stomata per unit area (stomata density) the more CO2 can be taken up, and the more water can be released. Thus, higher stomata density can greatly amplify the potential for behavioral control over water loss rate and CO2 uptake.