Page "Phloem" Paragraph 22

While movement of water and minerals through the xylem is driven by negative pressures ( tension ) most of the time, movement through the phloem is driven by positive hydrostatic pressures.

This process is termed translocation, and is accomplished by a process called phloem loading and unloading.

Cells in a sugar source " load " a sieve-tube element by actively transporting solute molecules into it.

This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube.

In sugar sinks, cells actively transport solutes out of the sieve-tube elements, producing the exactly opposite effect.