Xylem sap is responsible for the upward pull of nutrients and minerals from the roots. The root pressure in the xylem sap is the pressure caused by the continuous influx of water and nutrients from the soil. The xylem vessels are very narrow, this causes the easier upward pull of the xylem sap constituents. They also have a capillary action, this generates a suction pull and creates a vacuum. Due to this vacuum, there is minimal loss of water due to evaporation and transpiration. The xylem transport is more difficult since it is against gravity, going against gravity is tougher since the force of gravity needs to be overcome. Hence, the capillary flow in the xylem is more favourable!