The primary tissue of higher plants composed of thin-walled cells that remain capable of cell division, even when mature. Parenchyma constitutes the greater part of leaves, roots, the pulp of fruits, and the pith of stems. They are fundamental plant tissues as opposed to more highly differentiated tissues. In animals, the parenchyma constitutes the essential functional part of an organ, as contrasted with the organ's connective tissue, nerves, and blood vessels.