I learned many theories of root and shoot apical organization. Histogen theory, Haberlandt's division of eumeristem, Tunica corpus theory and cyto histological zonnation theory. But which is the actually relevant one that explains what is happening in apical meristems really? In my text book it said histogen theory is most valid for root Apex organization and Tunica corpus theory for shoot Apex organisation. But at the end, there is this diagram which marks protoderm within root apical meristem. I don't understand, why are they showing Haberlandt's division in figures if histogen theory is most relevant for root apical meristem? I'm really confused. Which is the actual theory that explains what happens in the apical meristems?