Definition of PALISADE LAYER. : a layer of columnar cells rich in chloroplasts found beneath the upper epidermis of foliage leaves —called also palisade mesophyll, palisade parenchyma, palisade tissue — compare spongy parenchyma.See palisade layer defined for kids.ADVERTISEMENT.efinition of PALISADE LAYER. : a layer of columnar cells rich in chloroplasts found beneath the upper epidermis of foliage leaves —called also palisade mesophyll, palisade parenchyma, palisade tissue — compare spongy parenchyma. See palisade layer defined for kids. ADVERTISEMENT.