(i) Mass flow
(ii) Source is that part of the plant, which synthesises the organic food, i.e., the leaves or it is the part of the plant where organic substances are loaded into the phloem.
Sink is that part of the plant, to where organic substances have to he transported for use.
(iii) At the source, when sugar enters the phloem, a hypertonic condition is created; water enters the phloem by osmosis and a higher osmotic pressure is developed.
So, the sap moves towards regions of lower osmotic pressure.