Final answer:
The openings in the epidermis of a leaf that allow for gas exchange, called stomata, are part of the dermal tissue, which is primarily made up of epidermal cells and guard cells.
Step-by-step explanation:
The plant tissue that has openings in the epidermis on the underside of a leaf, called stomata, where gases are exchanged is the dermal tissue. The dermal tissue consists primarily of epidermal cells that form a single protective layer covering the plant. Within this layer, the epidermis of a leaf contains the stomata, which are important for the process of photosynthesis, as they regulate the exchange of gases by opening and closing via the actions of surrounding guard cells. These stomata are critical in the uptake of carbon dioxide and the release of oxygen and water vapor during this process.