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A theoretical and experimental analysis of the systematic errors in the measurement of leaf temperature with thermocouples has been made. These errors are due to radiative heat exchange with the environment and to convective heat exchange with the ambient air. The magnitude of the error depends on the relevant temperature differences and on the resistance to heat transport between the object and the junction. The irreproducibility of this resistance causes the large variability in the measured temperatures. Measurement errors up to 7°C or to 30 %of the temperature difference between object and environment are found with thermocouples of 0.1 mm diameter. The radiation and convection error can be minimized by a reduction of the wire diameter and a careful positioning of the junction against the leaf surface with a slight pressure permanently exerted on it.• 339th Communication of the Laboratory of Plant Physiological Research.

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An optical planimeter is described in which the light beam from a small source is collimated and subsequently focused on a photocell. It is shown that the area of even a thin leaf placed in the collimated part of the beam is measured accurately without the need for a correction for transmitted light. The instrument features linear response, digital read-out, high speed, good stability, five ranges of 50 up to 1000 em', and an accuracy of 1% of the range.

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