Solution flow in tubular semipermeable membranes.

Solution flow in tubular semipermeable membranes was studied as a model for assimilate transport in sieve tubes. A mass flow of solution was demonstrated both in closed turgid tubes and in open tubes without turgor pressure. These results can be explained in terms of hydrostatic and osmotic pressure differences across the semipermeable membrane without consideration of a decrease in hydrostatic pressure along the direction of solution flow. A theoretical model based on nonequilibrium thermodynamics is developed that is in fairly good quantitative agreement with the experimental results. Münch's original experiment demonstrating solution flow is analyzed and shown not to depend on a gradient of hydrostatic pressure but rather to depend on the same driving forces operative in these experiments. On the basis of these findings a "volume-flow" mechanism of phloem transport is proposed.