Measurements of the rates of fluid exudation from excised roots of onion, and determinations of the osmolalities of the exudate fluids are found to be adequately related by the simplest model in which the rate of fluid exudation is proportional to the osmotic pressure difference between the exudate fluid and the bathing solution. The mean hydraulic conductivity of roots produced by unsprouted onion bulbs is 0.44 × 10−6 cm s−1 atm−1 and of roots from sprouted bulbs is 0.91 × 10−6 cm s−1 atm−l.