I think you are right. A callus is like a mass of undifferentiated cells (analogous to human stem cells). All cells in our bodies have the same DNA, but different tissues express some genes more than others, which is why some cells are liver cells and some are bone cells or lens cells. When undifferentiated cells (stems cells or meristems/calluses) are exposed to certain chemicals, they will differentiate into specific tissues (in plants, they are vascular tissue, leaves, roots, etc) that are determined by what the chemicals/hormones are. So a PLB, I think, is derived from a callus that has started the process of differentiation.
And the reason why not all cells differentiate into the exact same tissues (if they are in the same medium) is because the callus/stem cells will produce the differentiating hormones themselves but concentrate all the hormone on one side of the cell, so when the cell divides, one half will still be a stem cell and the other half will start differentiation.
Last edited by neophyte; 02-22-2019 at 12:21 PM..
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Definition of Callus versus PLB from Leaf Culture
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