Q.Mass flow hypothesis explains the process of

Munch's mass flow hypothesis (first proposed in 1930 and elaborated by Crafts in 1938) is the currently accepted explanation for phloem translocation: organic solutes move in bulk, as a mass flow, from a region of high osmotic pressure (the source, e.g. photosynthesising mesophyll) toward a region of low osmotic pressure (the sink), driven purely along a turgor-pressure gradient. Munch demonstrated the principle with a simple physical model: two chambers, A (holding concentrated sugar solution) and B (holding dilute sugar solution), each bounded by a semipermeable membrane and connected to each other by a tube T, the whole system immersed in a reservoir of water. Chamber A, being hypertonic relative to the reservoir, draws water in by endosmosis, which raises its turgor pressure; this rising turgor pressure then forces a mass flow of the sugar solution from A to B through tube T, and the flow continues until both chambers reach an isotonic state and the system becomes inactive again - though adding fresh sugar solution to A restarts the flow. In the living plant, mesophyll cells of the leaf (analogous to chamber A, the source) draw water osmotically from the adjoining xylem and build up turgor pressure, which drives the bulk flow of soluble organic solutes through the phloem sieve tubes (analogous to tube T) toward the lower-turgor cells of stem and root (analogous to chamber B, the sink), where the solutes are consumed, converted to an insoluble form, or have their excess water …