When transpiration is low, active transport of ions into the xylem decreases the stele's water potential and causes water flow into the stele. The water droplets escape through specialized structures called hydrathodes which relieve the pressure caused by more water entering the leaves than is lost by transpiration. Root pressure is not the major mechanism driving the ascent of xylem sap. Cannot keep pace with transpiration. Can only force water up a few meters.
Transpirational pull depends upon the creation of negative pressure.
Gaseous water in damp intercellular leaf spaces diffuses into the drier atmosphere through stomata. The lost water vapor is replaced by evaporation from mesophyll cells bordering the airspaces. The remaining water film, adhering to the hydrophilic cell walls, retreats into the cell wall pores. Cohesion in this surface film of water resists an increase in the surface area of the film - a surface tension effect.