Evidence for mass flow hypothesis

• If a ring of bark (which includes the phloem, but not the xylem) is removed from a woody stem, a bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring
• Sugar travels to many different sinks, not just to the one with the highest water potential, as the model would suggest
• Pressure in the phloem can be investigated using aphids. The sap flows out quicker nearer the leaves than further down the stem - this is evidence that there's a pressure gradient
• If a metabolic inhibitor (which stops ATP production) is put into the phloem, then translocation stops
• The radioactive carbon will be incorporated into organic substances produced by the leaf, which will be moved around the plant by translocation
• The movement of organic substances in a plant can be tracked using a radioactive tracer such as radioactive carbon (14C)
• The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate
• Transport in Plants

  1. Translocation of solutes in plants can be modelled in an experiment using radioactive tracers

  2. Supplying part of a plant with an organic substance that has a radioactive label, such as carbon dioxide containing the radioactive isotope 14C

  3. Radioactively-labelled CO₂ can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf
• The results demonstrate the translocation of substances from source to sink over time
• Photosynthesis produces glucose which is converted to sucrose for transport around the plant