According to histogen theory, as proposed by Hanstein, the root and shoot apices have three distinct meristematic regions, called as histogens. These are namely dermatogens, periblem and plerome. The dermatogens makes uniseriate epidermis by anticlinal divisions, periblem forms cortex and plerome serve as progenitor of stele (pericycle, vascular bundles, pith rays or medullary rays and pith).