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We consider a thermodynamic state of a solvent and solution separated with anelastic semipermeable membrane in a box with a constant volume and therelevance of this simple model for the water uptake in tall trees. Undermoderate concentrations of a solute, the solution and solvent are under thepositive and negative pressure, respectively. In the case of the soft membranethe pressure difference between the compartments with the solvent and solutionis given by van't Hoff equation. A state of the negative pressure is not stableand after some time cavitations transform the solvent into the state ofcoexisting liquid and bubbles of saturated vapor. The pressure differencebetween the solvent and solution decreases and the membrane relaxes restoringthe liquid phase in the compartment with solvent. In this way the solventoscillates between the tensile state and the coexisting state of liquid andbubbles of saturated vapor. The xylem and phloem, the main vascular systems intrees, are coupled with ray cells. Assuming that a sap in these systems is keptunder the constant volume the osmosis between the xylem and phloem with raycells sustains the negative pressure of the xylem sap. Due to the osmosiselastic energy stored in walls of tracheary element could repair cavitations.In this way both water transport in tall trees and cavitations repair intracheary elements are related to the osmosis under constant volume. A possibleexplanation of two long standing problems in tree physiology, water uptake intall trees and cavitations repair, are offered within this model.

Osmosis at constant volume and water uptake in tall trees