A condensation type porometer for field use.

A double cup transpiration porometer is described which simultaneously measures the resistance to water vapor diffusion from the upper and lower epidermises of a leaf. In use the cups are dried, the porometer is clamped on the leaf, and the transit time required for water vapor to condense on a cold (0 C) reflective surface in each cup is recorded. Since the sensing element remains at constant temperature, only transit time and leaf temperature are required to calculate leaf resistance.