A very simple and inexpensive device is described allowing continuous, environment independent measurement of the ultradian rhythm period of shoot movement. The ultradian rhythm is monitored by the interruption of a 940 nm light emitting diode generated light beam and a phototransistor. The results show that the period is constant under stable conditions, but is strongly dependent on temperature. Conversely, it is independent of short term (within 1 day) light conditions (presence or absence) and variations of relative humidity.