What is mesophyll tissue ? How are they differentiated?

Mesophyll tissue is the ground tissue of the leaf and lie between the two epidermal layers along with veins. It is a parenchyma tous green tissue and is usually differentiated into two regions-the upper palisade parenchyma and the lower spongy parenchyma.
(a) Palisade parenchyma : It consists of 1-3 layers of columnar, closely packed cells. The cells of palisade parenchyma contain abundant chloroplast and is the main photosynthetic tissue of the plant.
(b)Spongy parenchyma : It consists of oval, rounded or somewhat irregular cells with large intercellular spaces. The cells of spongy parenchyma contain few chloroplasts.