Sucrose at the SOURCE diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells, Active transport is used to actively load the sieve tubes of the phloem with sucrose, This lowers the water potential inside the sieve tubes, so water enters the sieve tubes by osmosis from the xylem and companion cells, This creates a high pressure inside the sieve tubes at the source end of the phloem, At the sink end, solutes are removed from the phloem and used up in respiration or turned to starch, This increases the water potential in the sieve tubes so water leaves by osmosis into respiring cells/xylem, This lowers the hydrostatic pressure inside the sieve tubes at the SINK end, This all results in a pressure gradient; a high hydrostatic pressure at the source and low at the sink., Therefore, there is a mass flow of sucrose solution down its hydrostatic gradient from the source end to the sink end.

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