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The permeability to water of whole detopped root systems of five herbaceous species, all grown under the same conditions, was determined by the solution-change method of Arisz, fielder and van Nie (1951). Measurements of root dimensions allowed permeability to be expressed per unit of (a) root surface area, (b) endodermis area, (c) root volume, and (d) root length. By any of these four methods of calculation there were large differences in permeability between the species. Per unit surface area the permeabilities were: broad bean, 0.54; dwarf bean, 0.56; sunflower, 0.71; maize, 2.2; tomato, 6.1 nm second[SUP-1] bar[SUP-1]. These differences cannot be explained by any obvious difference in root morphology. The permeability values fit with two previously proposed theories; (1) that much of the root resistance lies in the endodermis; and (2) that the resistance of the soil in the rhizosphere is small compared with that of the plant until the soil has dried to near or beyond the permanent wilting point. [ABSTRACT FROM AUTHOR]