Plant callus is a growing mass of unorganized plant parenchyma cells. In biological research and biotechnology, callus formation is induced from plant tissue samples (explants) after surface sterilization and plating onto tissue culture medium in vitro (in a closed culture vessel such as a petri dish). The culture medium is supplemented with plant growth hormones, such as auxin, cytokinin, and gibberellin, to initiate callus formation or somatic embryogenesis or morphogenesis.