A comparison was made of the morphological development of the primary root systems of aerated and non-aerated two weeks old bean plants (Phaseolus vulgaris L.). Elongation of the tap root axis was analysed with regard to cell division and cell extension. The development of non-aerated root systems proved to be directly, locally inhibited in the tap root. Moreover, indirectly, the development of the secondary roots was stimulated. It is shown that branching density in itself is no valid criterion of branching activity.