Plant callus is usually derived from somatic tissues. Remodelling of lamellar bone results in healing without callus formation. The saguaro creates callus tissue on the wound. In grafting, auxin promotes callus tissue formation, which joins the surfaces of the graft together. A callus is mass of undifferentiated parenchymatous cells. In rice, presumably, a condition which is favorable for scutellar callus induction induces necrosis too. Gautheret and Nobecourt were able to maintain callus cultures of carrot using auxin hormone additions. Immediately before the ocellar callus are two preocellar bristles. Retrorse barbs can be found on the callus, the lemmas, and the awns. Plant hormones are used to initiate callus growth. A third method is propagation by leaf by allowing the formation of a callus. There is a large yellow callus near the base of the labellum. Weightlifters commonly experience callus on the upper-palm area due to repeated friction. Sometimes a callus occurs where there is no rubbing or pressure. Typically, the wounds on the leaves are left to dry and callus over.