Munch mass flow hypothesis:

Munch’s “Mass Flow” Hypothesis- Munch (1930) proposed that the soluble food material in the phloem shows mass flow. Mesophyll cells of leaves synthesize sugars due to which the OP of these cells increases resulting in the absorption of water through the xylem. Now, the TP of these cells also increases towards the upper side and produces mass flow in the protoplasm of the sieve tube towards the lower side. Thus, the sugars move downwards into the roots where they are utilized during respiration and growth and their conc. Lowers down. This process continues.

Objection:

• The main defect of the Munch- hypothesis is that it explains only the unidirectional downward flow of soluble food materials.

• Mass flow is purely a physical process but phloem transport is an active process and requires energy.

• Saline content and other fibrils of the sieve tube reduce the speed of flow even under high pressure.