A callus is a mass of plant tissue that develops when cells from an explant or wounded area are stimulated to resume division under suitable culture conditions. The starting material may be a leaf, petiole, stem, root, bud, shoot tip, or embryo. When the explant receives an appropriate combination of nutrients, light, temperature, and plant growth regulators, certain groups of cells begin dividing and gradually form a tissue mass that may appear white, cream, yellow, green, o