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21. The sucrose and sap as a whole flow towards the stem and root in the sieve tube cells of the phloem because as water flows into the upper sieve tube cells, it creates a positive pressure or force that pushes the sap downward. This force is further increased by lowering the pressure in the sieve tube cells at the roots by actively transporting sucrose into the cortex cells, and by allowing water to diffuse into the cortex or xylem.
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