In the pressure driven movement of phloem sap, where does the water that raises the pressure come from?

1. Sucrose is actively loaded into the sieve-tube elements, raising the solute concentration there. 2. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. 3. The positive pressure that results pushes water and solutes down the pressure gradient towards the sink. 4. At the sink, sucrose is unloaded and the water returns to the xylem vessels. 5. Vapour entering through stomata would be water leaving rather than arriving, so that option reverses the flow. 6. Sink cells receive sap rather than supply the water that pressurises the tube. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.5 Transport of Water and Solutes in Plants_