Auxins and cytokinins together cause cell division in callus tissue culture. More auxin and less cytokinin causes root initiation whereas less auxin and more cytokinin cause bud differentiation. This is an example of synergetic action.
Auxins are of two types :
(1) Natural auxins : These are naturally occuring auxins, isolated from plants and are regarded as phytohormones e.g. IAA. NAA etc.
(2) Synthetic auxins : These are synthesized by artificially e.g. IBA 2,4-D etc.