plant callus A portion of plant tissue that consists of mass of undifferentiated plant cells. Consists primarily of parenchyma cells but may contain other cell types as the callus begins to differentiate. May be formed as a result of wounding or may develop in culture. portion of plant callus tissue A portion of plant tissue (PO:0009007) that consists of mass of undifferentiated plant cells (PO:0009002). Consists primarily of parenchyma cells but may contain other cell types as the callus begins to differentiate. May be formed as a result of wounding or may develop in culture. plant anatomical entity