A Working Model of a Sieve Tube

A working model of a sieve tube is described, based upon the Pressure-Flow Hypothesis. The flow of solution in the sieve tube is envisaged as being due to the joint influence of an axial turgor pressure gradient and a lateral water potential difference which causes an intake of water along the tube. In this second respect the model differs from that originally suggested by Münch. The model comprises a length of dialysis tubing rendered semi-permeable, incorporating capillary resistances at regular intervals. A sucrose solution is pumped into one end and collected at the other, the whole being submerged in water. Turgor pressures and concentrations along the model were recorded at intervals; thus the approach to the steady state was followed. In the steady state 14C-sucrose was introduced to the solution being pumped and its approach to a steady distribution in the model was followed. Important conclusions reached are that the Munch Pressure-Flow Hypothesis implies a pressure profile convex upwards, a velocity increase down the phloem, and an exponential fall in tracer concentration without its lateral leakage.