SummaryThe relative turgidity technique consists in comparing the initial and turgid water contents, on a percentage basis, of disks punched from leaves, the turgid water content being obtained by floating the disks on water.Three main sources of error associated with flotation are recognized: (1) changes in dry weight of the disks; (2) continued increase in water content after the attain· ment of full turgidity; (3) injection of the intercellular spaces at the cut edges of the disks.An examination of these sources of error led to the following conclusions concerning them:(1) By regulating the light intensity approximately to the compensation point, dry weight changes can be reduced to unimportant proportions. This obviates the necessity of taking duplicate samples since the final dry weight can be used for calculating both the initial and turgid water content of the disks.(2) Water uptake by floating leaf disks can be divided into two phases, phase I in response to the initial water deficit, and phase II the continued uptake, due to growth. The aim of the technique is to measure phase I alone. Metabolic inhibitors eliminated phase II but their use in the technique is unpractical (anaerobiosis) or objectionable (potassium cyanide). Low temperature (3°C) eliminated phase II but reduced phase I which itself appears to be divisible into more than one phase.It was found that for Ricinus communis L. full turgidity was attained in 4 hr and in this period phase II increase did not occur. A similar period appears to be suitable for a wide range of material. This short flotation time also reduced any danger of dry weight changes.(3) Injection errors were measured and found to be negligible for Ricinus.For Sambucus nigra L. they were considerable. Evidently the magnitude of this error varies with the species concerned.Plastic flow or irreversible contraction of the cell walls of turgid or wilted disks respectively was not found to be of importance.