| Summary: | <l type="ord"> A method has been developed to measure the hydraulic conductivity of the wall of the internodal cell of Nitella flexilis . The rate of water penetration through the cell wall varies linearly with the hydrostatic pressure difference between the two sides of the wall, showing that water permeability of the cell wall remains independent of the pressure difference applied. Water permeability of the cell wall is inversely proportional to its thickness It is 30μμ min −3{dot} atm −3 when the thickness of the wall is 10 μ. Water permeability of the cell wall is the same for inward and outward water flow. The polar water permeability of the entire membrane system (wall and protoplasmic part) of the living cell demonstrated by KAMIYA and TAZAWA ( 1 ) is, therefore, due to the living protoplasmic part. The ratio of the inward to outward permeability constants of the protoplasmic layer alone is higher than that of the entire membrane system composed of protoplasmic layer and cell wall. </l>
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