Curves relating the viscous conductivity, Sv sec/cm, to the diffusive conductivity, So cm-1, have been plotted for leaves of sugar beet and potato with defined stomatal concentrations and dimensions. The relationships and the theory of porometers have been used to obtain estimates of diffusive conductivity from readings taken with a portable porometer, and determinations made with an infiltration technique on leaves of both species.
Both techniques have been used to follow the hourly variation in diffusive conductivity of leaves of each species growing under field conditions. Estimates of S u varied between 0.1 and 7.9 cm -1, and between 0.3 and 14.8 cm-1 for leaves of potato and sugar beet, respectively, corresponding to pore widths of approximately 0.1 p and 5 # for both species.
Changes in SD could not be related satisfactorily to changes in the level of radiation, nor could they be explained by fluctuations in the relative turgidity of the leaves. Mean daily conductivity was not related to mean daily radiation nor potential transpiration. SD varied greatly with the soil water deficit.
The effect of changes in the level of soil water deficit on the actual transpiration rate relative to the potential rate is described. When deficits are of the order 20 cm, actual transpiration rates may be reduced by 60 ~o and 25 ~ of the potential rate for potato and sugar beet, respectively.