Final answer:
The tiny openings in the leaf's epidermis that let plants exchange gases with the atmosphere are known as stomata, which are regulated by guard cells.
Step-by-step explanation:
Tiny openings in the epidermis of the leaf that allow plants to exchange gases with the atmosphere are called stomata. These stomata are crucial for the process of photosynthesis as they allow the intake of carbon dioxide and the release of oxygen and water vapor. The stomata are usually found on the underside of the leaf to minimize water loss and are controlled by structures called guard cells. These guard cells swell or shrink in response to environmental cues, thus regulating the opening and closing of the stomata to maintain a balance between gas exchange and water balance.