  We consider a thermodynamic state of a solvent and solution separated with an
elastic semipermeable membrane in a box with a constant volume and the
relevance of this simple model for the water uptake in tall trees. Under
moderate concentrations of a solute, the solution and solvent are under the
positive and negative pressure, respectively. In the case of the soft membrane
the pressure difference between the compartments with the solvent and solution
is given by van't Hoff equation. A state of the negative pressure is not stable
and after some time cavitations transform the solvent into the state of
coexisting liquid and bubbles of saturated vapor. The pressure difference
between the solvent and solution decreases and the membrane relaxes restoring
the liquid phase in the compartment with solvent. In this way the solvent
oscillates between the tensile state and the coexisting state of liquid and
bubbles of saturated vapor. The xylem and phloem, the main vascular systems in
trees, are coupled with ray cells. Assuming that a sap in these systems is kept
under the constant volume the osmosis between the xylem and phloem with ray
cells sustains the negative pressure of the xylem sap. Due to the osmosis
elastic energy stored in walls of tracheary element could repair cavitations.
In this way both water transport in tall trees and cavitations repair in
tracheary elements are related to the osmosis under constant volume. A possible
explanation of two long standing problems in tree physiology, water uptake in
tall trees and cavitations repair, are offered within this model.
