The lower epidermis of a plant leaf is composed of a single layer of cells. There are four layers of cells in a typical leaf: the upper epidermis, the palisade layer, the spongy layer, and the lower epidermis. The upper and lower epidermis are single layers that protect the leaf.
In addition to protecting the rest of the leaf, the lower epidermis contains the majority of the stomata on the leaf. Stomata are tiny pores in the bottom of the leaf protected by guard cells. The stomata are necessary to exchange gas with the outside environment. The plant uses stomata to take in carbon dioxide for photosynthesis and expel oxygen.