Currently, the best explanation of translocation is provided by Munch.
Figure 1.25 and the accompanying text presents a brief summary of this explanation , also knows as the pressure flow hypothesis:
At the source where sugar is produced during photosynthesis, the sugar is a actively transported into the sieve tubes of the phloem.
Water then flow into the sieve tubes by osmosis ( water may be drawn from the xylem vessels ) .
The pressure gradient created moves the sap through the phloem.
Where sugar is being used up or removed for storage , sugar is moved out of the phloem either by active or passive transport.
Water then moved out of the phloem by osmosis and is returned to the xylem from the phloem.