The tissues used to initiate callus formation depends on plant species and which tissues are available for explant culture. The cells that give rise to callus and somatic embryos usually undergo rapid division or are partially undifferentiated such as meristematic tissue. In alfalfa, Medicago truncatula, however callus and somatic embryos are derived from mesophyll cells that undergo dedifferentiation.[17] Plant hormones are used to initiate callus growth.