The pressure flow hypothesis, also known as the mass flow hypothesis, proposes that the movement of sap in the phloem is driven by a pressure gradient created by differences in osmotic pressure between the source (where sugars are produced) and the sink (where sugars are consumed or stored). This process involves the loading of sucrose into the phloem at the source, which leads to water entering the phloem by osmosis, creating a high-pressure zone. The sucrose is then unloaded at the sink, causing water to exit the phloem, thus creating a low-pressure zone.
