Abstract.Wphen there is no external source of water, plants can grow by mobilizing internal water from nongrowing tissues. We investigated how this internal water moves by measuring continuously and simultaneously the water potential (ψw) of soybean (Glycine maxL. Merr.) seedlings in the upper, growing stem tissues and the lower, non‐growing stem tissues. When external water was available to the roots, the stems grew rapidly and the ψwof the growing tissue was continually below that of the nongrowing tissue and the medium around the roots. This indicated that a growth‐induced gradient in ψwfavoured water movement from the external source to the growing cells. When the external source was removed, the ψwof the growing tissue remained constant for a time and the ψwof the nongrowing tissue decreased somewhat. Growth took place slowly as water was withdrawn from the nongrowing tissue but ψwgradients continued to favour water transport to the growing cells. On the other hand, if this internal source was removed by excision, growth ceased abruptly. In this case, the cell walls relaxed and the ψwof the growing tissue decreased by about 0.1 MPa instead of remaining constant. The ψwof the detached nongrowing tissues remained constant instead of decreasing. This indicates not only that water mobilization required attached nongrowing or slowly growing tissues but also that mobilization affected wall relaxation. Thus, ψwdifferences may mobilize internal water, may explain the continued growth of plants and plant parts removed from external sources of water, and may account for discrepancies in measurements of cell wall properties in gro