Transfer cells selectively pump ions out of the symplast into the apoplast so they may enter the xylem. This action requires energy. Water transported up from the roots must replace water lost by transpiration. This uptake force is called root pressure. Cannot keep pace with transpiration, can force water up only a few meters.
Process continues back to the tracheids causing water up take from the xylem sap.
Water goes fro tracheids to air following a water potential gradient. Cohesion and adhesion of water. H bonds of water, hydrophilic walls of xylem and small diameter of xylem aid in the movement of water up the tube. This pull decreases water pressure in the xylem causing the roots to take water from the soil. Turgid guard cells open the stomata, while flaccid cells close them.