(b) : The major physiological function of cytokinin is to enhance cell division. In tissue culture the undifferentiated mass of cells formed in the culture tubes is called callus. The callus may remain in the undifferentiated condition or differentiation may take place in this. If it is differentiated, then root and shoot may be formed. Skoog and miller had reported that cytokinins induce shoot formation and auxins induce root formation (figure).
Later however it was proved experimentally that - (i) High cytokinins/auxin ratio - Differentiation of shoot. (ii) Low cytokinins/auxin ratio-Differentiation of root. (iii) Intermediate cytokinins/auxin ratio Differentiation of both root and shoot. (iv) Intermediate cytokinins/low auxin - Callus formation.