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Student Introduction: Plant biologists spent many decades trying to understand the mechanisms behind transport of nutrients through seive tubes in the phloem. Proponents of mass transport believed the gradient of solute concentration along seived tubes would create a gradient of turgor pressure. The pressure gradient could be quite small (0.020 MPa per meter) but still be enough to drive the flow of nutrients (see Dixon's Paradox). However, the idea of mass transport seemed too simplistic for many plants whose cell-to-cell pressure gradient or speed of transfer did not match that predicted by Dixon's equation.

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