Mass flow Hypothesis

Glucose produced by photosynthesis is converted into sucrose which is actively loaded to the phloem from companion cells at the source.
Water potential inside the leaf phloem becomes more negative due to the high concentration of sucrose, so water moves by osmosis from neighbouring xylem vessels down a water potential gradient and into the phloem.
This cause a high hydrostatic pressure at the source end of the phloem, so water and dissolved solutes are passively forced towards the sink…

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