<Li> Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes - water and minerals . The phloem pressure can rise to several MPa, far higher than atmospheric pressure . Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure . </Li> <Li> Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant . This causes millions of minute menisci to form in the mesophyll cell wall . The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil . </Li> <Li> Root pressure: If the water potential of the root cells is more negative than that of the soil, usually due to high concentrations of solute, water can move by osmosis into the root from the soil . This causes a positive pressure that forces sap up the xylem towards the leaves . In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation . Root pressure is highest in the morning before the stomata open and allow transpiration to begin . Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus . </Li> <P> The primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits . Capillary action provides the force that establishes an equilibrium configuration, balancing gravity . When transpiration removes water at the top, the flow is needed to return to the equilibrium . </P>

The contents of the xylem sap in plant roots are