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A method is described by means of which the suction pressure of living material may be measured by observations on its equilibrium vapour pressure. It employs a very fine thermocouple whose tip is cooled by means of the Peltier effect, below the dew-point. It collects a minute amount of dew, and can thus be used as an exceedingly fine ‘wet-bulb’ thermometer. As such it is convenient to use, quick in action, and causes very little disturbance to the system under observation. The thermocouple is calibrated by means of salt solutions of known strength. The scale is fairly linear. The method possesses the advantages of giving the suction pressure by direct observation rather than by interpolation; of speed in the actual measurement; of not subjecting the tissue to immersion; and of permitting organs like leaves, otherwise difficult to handle, to be easily dealt with. It also opens possibilities outside the range of the conventional methods.
D. C. Spanner (Mon,) studied this question.