The mesophyl form the photosynthetic ground tissue of the leaf, and is sandwiched between the upper epidermis and the lower epidermis. The mesophyll cells are loosely arranged with many air spaces between them that facilitate gas exchange. In many plants, the mesophyll is divided into two layers: the palisade mesophyll which is the main site of photosynthesis and the more loosely and irregularly arranged spongy mesophyll layer, that functions primarily in gaseous diffusion within the leaf