Mesophyll:
The bulk of tissue lying between upper and lower epidermis (except vascular bundles) is called mesophyll. It consists of parenchymatous cells containing abundant chloroplasts and separated by lots of intercellular spaces. In some leaves (e.g., Hydrophytes), it is made up of one type of cells but In others the mesophyll is differentiated into palisade parenchyma and spongy parenchyma.
The palisade is made up of compactly arranged columnar cells elongated at right angles to the leaf epidermis. The spongy parenchyma, on the other hand, consists of loosely arranged cells (spherical,oval, and are irregularly arranged) with wide intercellular spaces. These spaces are interconnected and open into substomatal chambers to maintain gaseous exchange through stomata.
Besides palisade and spongy parenchyma, the mesophyll also possess different kinds of sclereids. For example, the leaves of Hakea possess osterosclereids (rod-shaped stone cells) among palisade. Some hydrophytes possess branched sclereids in their larger air spaces (Nymphaea) or sphaeraphides in specialized cells (e.g., Trapa bispinosa).
