Phloem transport

Ring girdling removes a complete ring of bark, which includes the phloem.
Sugars can no longer be translocated down to the roots, so the roots die first.

Sieve tube elements lack a nucleus at maturity, and their functions are controlled by the nucleus of the adjacent companion cells, to which they are connected by pit fields.
According to the pressure flow (mass flow) hypothesis, sucrose made in the mesophyll cells is moved into the companion cells and then into the living sieve tube cells by active transport. This is called phloem loading.
Because loading is active (it uses ATP), it is not a passive process.
Loading makes the phloem hypertonic, so water enters it from the xylem by osmosis; the companion cells do not supply water.
Hence the function of companion cells is loading of sucrose into sieve elements.

Food, mainly sucrose, is transported from the source (leaves) to the sink through the phloem.
The conducting elements of phloem are the sieve elements (sieve tube elements in angiosperms, sieve cells in gymnosperms and pteridophytes), which form long columns with perforated sieve plates in their end walls.
Companion cells help to maintain the pressure gradient and to load sugars, but the actual channel of transport is the sieve tube.
Tracheids conduct water, and transfusion tissue occurs in gymnosperm leaves for lateral conduction.
Hence food is transported through sieve elements.

The accepted mechanism for the translocation of sugars from source to sink is the pressure flow or mass flow hypothesis.
At the source (for example a leaf), sucrose is moved into the companion cells and then into the sieve tube cells by active transport, using carrier proteins and the energy of ATP. This is phloem loading.
Loading makes the phloem sap hypertonic, so water enters the sieve tubes from the adjacent xylem by osmosis and builds up a high turgor pressure.
This pressure pushes the sap in bulk through the sieve tubes towards the sink, where the pressure is lower.
At the sink, the sugar is again removed actively (phloem unloading), and water moves out.
Root pressure and transpiration pull are concerned with the ascent of water in xylem.
So translocation in sieve tube members is supported by mass flow involving a carrier and ATP.