The mass flow hypothesis of transport in phloem is as follows:

Mass Flow Hypothesis proposed by Munch in 1930 explains the movement of sap via the phloem. When there is a high concentration of sugar present in the source (which is where the food is prepared) then a diffusion gradient gets created between the sugar source and the sugar shrink where the sugar is stored. This is responsible for drawing water into the cells from the neighbouring xylem.When this happens, turgor pressure gets created (bidirectional movement).

A part of the glucose produced by photosynthesis is converted into sucrose. This is then transported actively to the companion cells and then to the sieve tubes via diffusion. The osmotic pressure moves the sucrose through the sieve tube towards the sink. When the sucrose reaches the sink, it is removed into apoplast. Water moves out of the sieve tube by osmosis establishing a pressure gradient. A low turgor pressure is created at the sink to maintain this gradient. Water passes back into the xylem later again.

Thus the mass flow hypothesis' mechanism is dependent on the turgor pressure as well as the osmotic pressure gradient between the source and the sink.