In plants, the epidermis layer functions like a covering of skin to protect the leaf, while vascular bundles carry water and nutrients through the leaf. The palisade mesophyll layer is responsible for most of the photosynthesis that occurs, while the spongy mesophyll layer underneath it aids in the exchange of gases (CO2 and O2) and water vapor. In which leaf tissues would one expect to find the highest density of chloroplasts?