The water in different parts of a plant forms a continuum throughout the plant body. This makes it possible to record changes in the water content as changes in thickness of any part of the plant. A leaf kept in darkness has been found to change its thickness to a sufficient degree for recordings of changes in transpiration from the rest of the plant. The rapidness of the changes makes it probable that they are mirroring the stomatal movements.
The method has been used for recording of influences of SO2 as air pollutant. It has a couple of advantages over direct measurements of changes in transpiration. One is that the measurement can be used without enclosing the plant in a cuvette. Another is that possibly occurring effects on the hygrometer are eliminated. The method has until now mainly been used for wheat plants but also woody plants as Pinus and Salix have been tested.