Final answer:
The layer below the palisade mesophyll is the spongy parenchyma, which contains loosely arranged cells and intercellular spaces. Both layers of the mesophyll contain chloroplasts.
Step-by-step explanation:
The layer below the palisade mesophyll is the spongy parenchyma. Below the tightly packed column-shaped cells of the palisade mesophyll, there are loosely arranged cells of an irregular shape in the spongy parenchyma.
The spongy parenchyma contains intercellular spaces that allow gaseous exchange between the leaf and the outside atmosphere through the stomata. These spaces also help the leaf float in aquatic plants.
Both layers of the mesophyll contain many chloroplasts, but the palisade mesophyll cells have more obvious chloroplasts.