Suggest why the presence of the sieve plate is often cited as evidence against the pressure-flow model for phloem transport

I think this is because the role of sieve plates is unclear, the mass flow would improve without them and therefore their presence goes against the mass flow theory. The mass flow hypothesis explains translocation which is the movement of sugars and amino acids in solution from a high pressure region to a low pressure region through the phloem sieve tubes. 

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