The permeability of unplasmolysed cells of beetroot, v. ‘Crimson Globe’, was determined from the rate of water loss of beet slices on placing in sucrose solutions having O.P. greater than the suction pressure of the beet. The absolute values obtained were about 0·7μ 3 water per μ 2 cell-surface per hour per atmosphere osmotic pressure difference, i.e. 0·7 μ/hr./atm. The permeability of similar beet cells plasmolysed within their cell walls was found to be about 13μ/hr./atm. The permeability of beet cells which had been plasmolysed and allowed to recover was shown to be approximately the same as that of unplasmolysed cells. The hypothesis is advanced that the increase in water permeability on plas-molysis is due to those parts of the plasma-membrane which had formerly been pressed against the micelles of the cell wall becoming free and able to take part in water transfer. The energy requirement for the maintenance of an excess hydrostatic pressure of five atmospheres within a cell by its vital activity was shown to be about one-tenth of the total respiratory energy released in freshly cut beet slices.