The photosynthetic tissue of a leaf is the palisade parenchyma, the upper layer of the middle layer of the leaf. It sits above the spongy parenchyma, the lower part of the mesophyll with far fewer chloroplasts. The mesophyll is sandwiched between the upper and lower dermis of the plant.
The mesophyll, where the palisade parenchyma is located, is also the site of the spongy parenchyma and the vascular tissues of the leaf. Between the mesophyll and the epidermis of the plant are collenchyma cells, which have variable cell wall thickness and, along with the vascular tissue, help maintain the leaf's structure.
The cells of the palisade parenchyma are arranged in an ordered grid, with each cell approximately square in cross section. The cells have relatively thin, flexible cell walls relative to other cells in the plant. While they share this feature with the spongy parenchyma, the spongy parenchyma are irregularly shaped with many gaps between them. This allows gases to pass from the stomata on the underside of the leaf. This is critical, since the process of photosynthesis requires the intake of carbon dioxide gas. It also produces oxygen gas as a waste product, which must be released. The vasculature of the leaf carries water to the cells, another crucial ingredient in photosynthesis.