Plant callus (plural calluses or calli) is a growing mass of unorganized plant parenchyma cells. In living plants, callus cells are those cells that cover a plant wound. The culture medium is supplemented with plant growth regulators, such as auxins, cytokinins, and gibberellins, to initiate callus formation or somatic embryogenesis. Callus initiation has been described for all major groups of land plants. Callus must form before roots can emerge.

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