The pressure flow or mass flow hypothesis:
When glucose is prepared at the source, it is converted to sucrose.
The sucrose moves into the companion cells and then into the living phloem sieve tube cells; through active transport. This process of loading at the source produces a hypertonic condition in the phloem.
Water; from the adjacent xylem; moves into the phloem, by osmosis. This results in an increase of osmotic pressure. It forces the phloem sap to areas of lower pressure, i.e. towards the sink . The osmotic pressure must be reduced at the sink.
Active transport moves the sucrose out of the phloem sap into the cells in the sink. Once the sugar is removed, the osmotic pressure decreases and water moves out of the phloem.
