The palisade cells are usually the most active photosynthesising cells in the typical leaf. They are close to the upper surface, and so receive high levels of light, while their lower ends are in contact with the air spaces of the spongy mesophyll and so can take in carbon dioxide. Their long thin shape and vertical position means that light entering the palisade cell can hit the chloroplasts without having to be weakened by travelling through too many cell walls.
