Question

explain Munch's mass flow hypothesis with merit and demerit

Answer 1

According to the Munch’s mass flow hypothesis phloem sap moves from sugar sources to sugar sinks through turgor pressure. Due to the high concentration of organic substances inside source cells such as leaves there occurs a diffusion gradient which causes inward movement of water into the cells. This causes the movement of food from source to sink.
Merits
There are a number of evidences in support of this theory like when a stem is cut a solution comes out of phloem indicating that there is pressure in the phloem sap.
Demerits
Munch’s hypothesis does not take into account the living nature of phloem.
The hypothesis believes that all substances being transported travel at uniform speed which is not true as amino acids and sugars travel at different speeds.