Question

mass of undifferentiated cells​

Answer 1

Plant species representing all major land plant groups have been shown to be capable of producing callus in tissue culture. ... The cells that give rise to callus and somatic embryos usually undergo rapid division or are partially undifferentiated such as meristematic tissue.

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Answer 2

The callus is mass of undifferentiated cells. The callus culture involves the differentiation of cells and consequently growth of differentiated cells into different organs of the plant.