Mesophyll cells constitute the main body of a leaf, occurring between the upper and lower epidermis. Typically, the leaves of temperate-zone plants have two layers of mesophyll cells, the palisade mesophyll on the adaxial (upper) side, and the spongy mesophyll on the abaxial (lower) side. The palisade mesophyll is a layer of densely packed, columnar cells which contain many chloroplasts. This layer is responsible for most of the photosynthesis of leaves. The spongy mesophyll is composed of large, often odd-shaped, photosynthetic cells separated from one another by large, intercellular spaces. The intercellular spaces apparently facilitate the exchange of photosynthetic gases.