Biology help - translocation

The sucrose in the leaf (the source), is actively transported into the phloem via the companion cells. This reduces the water potential in the phloem, so as a result water from the xylem moves down the water potential gradient into the phloem via osmosis. So now the hydrostatic pressure in the phloem is high. As the sucrose reaches the sink cells it is used for respiration or converted into starch. As the sucrose moves into the cells by active transport this lowers the water potential of the cells so water moves by osmosis too, thus reducing the hydrostatic pressure near the sink cells.

Hope this helps