This is a photograph of a leaf surface of a narrowleaf cottonwood (Populus angustifolia) taken under a light microscope (at...checking lab notebook for level of magnification...). All you need to get imprints of leaf surfaces is glass microscope slides, transparent scotch tape, and clear nail polish.
The prominent oval shapes in the photograph are stomata (singular: stoma). Stomata are pores that let stuff in and out of the leaf. In other words, they control the movement of gases in and out of the leaf (carbon dioxide used for photosynthesis and water out via transpiration). Variation in the size, the number, and the location (top or bottom of leaf) of stomata on leaf surfaces are a few ways that plants can control water loss, and thus are important to plant function.
Trivia: Do plants typically have a higher density of stoma on the top or the bottom surface of their leaves?