Movement of organic solutes or photo synthates from one part of the plant to the other is called translocation.
Munch used a model system to explain the translocation of solutes. Two Osmometers A and B were taken. ‘A’ filled with concentrated sugar solution and ‘B’ with dilute sugar solution. The Osmometers are made of semipermeable membranes. These are connected by means of a double bent tube ‘C’ containing water. The entire system is left in a trough D containing water. Water enters the osmometer A because of osmosis resulting in mass flow from A to C and finally B. The water from B is forced out because of pressure created due to mass flow.
This working model is comparable to the actual process occurring in plants, a pressure gradient exists between the source (comparable to A) where sugar is loaded (leaves) using ATP and the sink (comparable to B) where sugar is unloaded (storage organs). In the leaves, sugar enters the sieve tubes from companion cells which lower the water potential which pushes the sugar solution through the sieve tube-like water being pushed through a hose. At the sink, sugar is actively unloaded with the use of ATP, which increases the water potential and water moves out.
This, however, is the accepted model of phloem transport. The theory fails to explain
- Bidirectional flow of organic substances.
- The role played by phloem.
- The resistance offered by plasmodesmata and sieve plates.
- It regards the process as purely physical
merits
- Supply end always has a high sugar level
- Turgor decreases in defoliated plants.
