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. 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 regulators, such as auxin, cytokinin, and gibberellin, to initiate callus formation or somatic embryogenesis. Callus initiation has been described for all major groups of land plants. Callus induction and tissue culture Plant species representing all major land plant groups have been shown to be capable of producing callus in tissue culture. A callus cell culture is usually sustained on gel medium. Callus induction medium consists of agar and a mixture of macronutrients and micronutrients for the given cell type. There are several types of basal salt mixtures used in plant tissue culture, but most notably modified Murashige and Skoog medium, White's medium, and woody plant medium. Vitamins are also provided to enhance growth such as Gamborg B5 vitamins. For plant cells, enrichment with nitrogen, phosphorus, and potassium is especially important. Plant callus is usually derived from somatic tissues. 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. Plant hormones are used to initiate callus growth. After the callus has formed, the concentration of hormones in the medium may be altered to shift the development from callus to root formation, shoot growth or somatic embryogenesis. The callus tissues then undergo further cell growth and differentiation, forming the respective organ primordia. The fully developed organs can then be used for the regeneration of the new mature plants. Morphology Specific auxin to cytokinin ratios in plant tissue culture medium give rise to an unorganized growing and dividing mass of callus cells. Callus cultures are often broadly classified as being either compact or friable. Compact calluses are typically green and sturdy, while friable calluses are white to creamy yellow in color, fall apart easily and can be used to generate cell suspension cultures and