The accepted mechanism used for the translocation of sugars from source to sink is called the pressure flow hypothesis.
As glucose is prepared at the source, it is converted to sucrose, which is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
Water in the adjacent xylem moves into the phloem by osmosis.
As osmotic pressure builds up the phloem sap will move to areas of lower pressure and at the sink osmotic pressure must be reduced.
Active transport moves the sucrose out of the phloem sap and into the cells which will use the sugar converting it into energy, starch, or cellulose.
As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.
Loading of the phloem with sugar sets up a water potential gradient that facilitates the mass movement in the phloem.
Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates.
Cytoplasmic strands pass through the holes in the sieve plates, so forming continuous filaments.
As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem.
At the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates.
The loss of solute produces a high water potential in the phloem, and water passes out, returning eventually to xylem.
Girdling experiment was used to identify the tissues through which food is transported.
On the trunk of a tree, a ring of bark up to a depth of the phloem layer, can be carefully removed.
In the absence of downward movement of food the portion of the bark above the ring on the stem becomes swollen after a few weeks.
This simple experiment shows that phloem is the tissue responsible for translocation of food; and that transport takes place in one direction, i.e., towards the roots.