The loss of water (evaporation) by plant leaves is regulated by the stomata (pores) of the leaves, as CO2 absorption is necessary for photosynthesis. It is an important indicator for the physiological condition of the plants. The opening of the stomata can be interpreted as the resistance against gas diffusion (in s/cm) and is measured using the porometer. The porometer is fitted with an automated, fast measuring cycle which yields temperature compensated measuring results. Since the stomata are sensitive to the presence of the porometers sensor head, measurements are executed within less than 15 seconds. The instrument has a measuring range of 0 - 30 s/cm and is suitable for wide and narrow leaves. The instrument has an internal memory for approximately 1500 measurements, including notes. Data can be transmitted via an RS232 interface to a printer or PC. The instrument is provided with a clearly readable display and function keys. In preparation of a series of measurements, the porometer can be calibrated fast and easy by using software and a calibration plate. The porometer is supplied including sensor head, calibration plate, RS232 cable, software, charging apparatus and carrying bag.