• The mechanism of transport that occurs within the phloem is still debated by scientists
• The mass flow hypothesis suggests that there is a translocation of sucrose and other sugars within the phloem occur via a?continuous unidirectional flow?of water and dissolved nutrients
  • The direction of flow is from the source (where sugars are produced) to the sink (where sugars are respired)

   

• Many experiments have been conducted to try and determine whether the mass flow hypothesis is an accurate representation of what occurs within the phloem
• If the mass flow hypothesis is correct then in any one sieve tube there should be a bulk flow of phloem sap in?one direction,?travelling at the?same rate.?This should also be occurring at the?same time?in most sieve tube elements.

Evidence supporting the mass flow hypothesis

• There are several observations that support the hypothesis
• When the phloem sieve tube is?punctured?phloem sap oozes out
  • This suggests that it is?under pressure

   

• Phloem sap taken from near a source has a higher?sucrose concentration?than sap taken from near a sink
  • This suggests that different water potentials would result in?osmosis?into/out of the sieve tubes at those two locations

   

• When a?plant virus is applied?to leaves that are well-lit the virus can be observed moving down the phloem towards the roots
  • This demonstrates the bulk flow of substances in one direction

   

• When the virus is applied in the?dark?it is not transported
  • This suggests that for translocation to occur photosynthesis and the production of sucrose is required in the source tissue

   

Evidence contradicting the mass flow hypothesis

• The?rate of translocation?of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose
  • The mass flow hypothesis states should be flowing at the same rate

   

• Some scientists have conducted experiments that detected different substances (within the same sieve element) moving in?opposite directions
  • The mass flow hypothesis states everything should be flowing in one direction

   

• It has been suggested that some sieve tubes?translocate at different times
  • The mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leaves