Evaluation of Mass Flow Theory

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Mass Flow Theory to Explain Translocation


  • sap is released from the sieve tubes when they are cut, which suggests that not only do they carry sugar, there is a pressure in the tubes that causes it to flow
  • sucrose concentration is greater at the leaves than the sinks, which suggests that there must be a mechanism to get it from the leaves to the sources
  • there is a downward flow in the phloem during the day, which stops by night. This suggests that translocation is dependent on light - photosynthesis creates lots of sugars which need to be transported away
  • when a plant's metabolism is inhibited, translation also stops which suggests that in one or more stages of translocation, ATP is needed
  • companion cells have many mitochondria and readily make lots of ATP. Cell specialisation would suggest that these cells have evolved this way for a purpose


  • the function of sieve plates is unknown, implying that Mass Flow Theory is not comprehensive
  • not all solutes move at the same rate in solution, which goes against the theory, which claims that solutes move at a rate at which they are needed, not on which they individually are capable of
  • sucrose moves at a standard speed, and does not appear to change dependent on concentration gradient
  • sieve plates should in theory actually inhibit mass flow as their perforated surfaces should inhibit the desired pressure changes to make the solution travel


Mass Flow Theory is the most widely accepted and the most likely explanation for translocation. However, little is known about the specific jobs of the sieve elements.


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