A Differential Psychrometer for Continuous Measurements of Transpiration.

Transpiration is frequently measured by enclosing plants, or portions of plants, in transparent chambers and determining the change of humidity of a stream of air before and after passing through the enclosure (2, 4). Although this procedure should not be used to estimate probable transpiration rates under natural conditions, unless adequate precautions are taken to ensure that the microenvironment within the chamber does not differ significantly from that in the reference locations or that its specific characteristics can be measured and appropriate corrections made (9), it is particularly suitable for specific studies of the transpiration process. Recently, in developing an apparatus for simultaneous and continuous determinations of transpiration and photosynthesis involving differential airstream measurements of waiter vapor and CO2 concentration, Bierhuizen and Slatyer (1) incorporated a differential psychrometer, based on a design of Wylie (10). This instrument has been found to be particularly suitable for transpiration research (7, 8) and its characteristics and performance are outlined below.