On the heat-pulse method for the measurement of apparent sap velocity in stems.

The velocity of a heat pulse moving through the xylem of a stem is generally not the same as the sap velocity. One can measure the sap velocity by carefully characterizing the shape of such a pulse. In much contemporary work the phase relations of sap flow are more important than the actual velocity. It is shown that the measurement of the time for the peak of a heat pulse to reach a sensor can characterize an "apparent heat-pulse velocity". A theoretical analysis shows that this velocity is expected to be in phase with the sap velocity.A temperature-compensated system for the measurement of apparent sap velocity in the stem of a cotton plant is presented. Output variations caused by changes in the ambient air temperature are eliminated through use of opposing thermistors in a bridge circuit. One thermistor was used as a sensor while the other served as compensator for ambient temperature changes.