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Abstract It is widely believed that there is a growing body of evidence that sieve plate pores in functioning phloem are open and quite free from obstruction. The arguments used to establish this idea are critically examined one by one and the conclusion is reached that it has very little foundation in published work. On the contrary, the view that sieve plate pores are delicately-occluded with P-protein in the angiosperms has much more evidence to substantiate it. Since ‘open’ and ‘occluded’ are physiologically defined this means that the pressure-flow hypothesis must be regarded as seriously in doubt.