A falling mercury plug draws air through a leaf cup in series with a standard capillary resistance bridged by a mercury manometer. Readings on the manometer correspond to leaf conductances. Thus no timing is necessary. The cup is a short length of latex rubber tubing (mternal diameter 2·5 mm) held in position by a light clip. The apparatus can be held and operated in one hand leaving the other free to manipulate the porometer chp. No levelling is necessary. The poro-meter is contained in a case 24 × 6 × 3 cm and is easily portable. Using the apparatus itself, capillaries are readily standardized in units of conductance expressed in terms of rate of flow of air through the capillary per unit pressure difference (mm3/min/rnm Hg). Using a 2·5 mm cup the highest stomatal conductances measured were around 200 units. Three standard capillaries are incorporated in the porometer allowing approximately a 500-fold change in leaf conductances to be covered.
The performance of the instrument on Sambucus nigra, Vaccinium vitis-idaea and Poa annua is demonstrated and some of its advantages and weaknesses discussed.