Those minerals and water which move into the apoplast are free to enter the tracheids and xylem vessels. The shoot depends upon an efficient delivery of its water supply. Xylem sap flows upward at 15m per hour or faster. Xylem vessels are close to each leaf cell, because veins branch throughout the leaves. Water transported up from roots must replace that lost by transpiration .
Transpiration is the evaporation of water from the aerial parts of a plant.
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.