Plasmodesmal permeability describes how readily specified material moves between plant cells through plasmodesmata under stated conditions. It depends on the transported substance and the state of the connections; a count of channels alone does not specify exchange. Passage through a plasmodesma is also a different route from crossing a lipid cell membrane.
The distinction is experimentally accessible. In developing cotton fibres, investigators used a fluorescent tracer and confocal microscopy to follow access between the seed coat and growing cells. Access was observed early, blocked around day 10 and observed again around day 16. Tests of movement in the reverse direction provided a complementary functional comparison.[ref]
The control of single-celled cotton fiber elongation by developmentally reversible gating of plasmodesmata and coordinated expression of sucrose and K+ transporters and expansin by Y. L. Ruan and others. Abstract; Figures1–2: developmental changes and reverse tracer movement
An original toy model illustrates why channel number is insufficient. Ten connections each carrying two tracer units per unit time give 20 units per unit time altogether. Twenty connections each carrying half a unit give 10. These invented values compare total transfer in an equal-channel model; they are not a universal transport law.
Structure must therefore be paired with function. A 2017 electron tomography study found post-cytokinesis plasmodesmata with no visible open cytoplasmic sleeve, yet detected macromolecular diffusion. The imaged arrangement and the movement assay supply different evidence about the same interface.[ref]
Architecture and permeability of post-cytokinesis plasmodesmata lacking cytoplasmic sleeves by William J. Nicolas and others. Abstract: membrane architecture and retained macromolecular transport
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