Young plants of Phaseolus vulgaris were grown with their hypocotyls immersed in a nonaerated nutrient solution. Blocking of the hypocotyl with paraffin oil resulted in the deterioration of the lower part of the hypocotyl and a shift of the development of adventitious roots from the lower part to the upper part of the hypocotyl. No inhibiting effect of the paraffin treatment on the growth of roots and leaves was observed in plants which were immersed in aerated nutrient solutions or in plants, the root bases of which were at water level on nonaerated solutions. The effect of the paraffin treatment is attributed to the blocking in the hypocotyl of the gas-exchange between the shoot and the roots. The importance of an internal transport of gases is briefly discussed.