Abstract
The porometer measures changes in stomatal resistance to flow by drawing an air stream through the stomata into a porometer cup, thence through a variable capillary resistance to a constant pressure aspirator. A manometer is inserted between the porometer cup and the capillary resistance and change in stomatal resistance is measured by change in level of the manometer. The stomatal resistance can be calculated in capillary units of resistance. A modification for automatic recording is described. Results showing some known stomatal effects are presented, as well as a continuous automatic record of stomatal movement over 7 days in alternating periods of 3 hours of light and of darkness. The porometer is particularly sensitive to change in aperture when the stomata are wide open; practically instantaneous readings can then be made.

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