EFFECTS OF CIRCUMNUTATION AND PASSIVE BENDING ON THE INITIAL-STAGES OF GRAVITROPISM IN PEA STEMS

The influence of circumnutation and passive stem bending on gravitropi sm of 9- to 10-d-old light-grown pea seedlings (Pisum sativum L.) was studied using time-lapse photography. Plants placed horizontally witho ut regard to circumnutation exhibit erratic movements, but if they are stimulated when in the same phase of the circumnutational cycle these irregularities disappear. In plants turned vertically after a short p eriod of horizontal stimulation, the initial bending is due more to ci rcumnutation than to gravitropism. The initial response to gravity in horizontally placed plants is a downward movement of the stem. The kin ematics of this movement suggests it is due both to elastic and plasti c passive stem deformation and is not a positive gravireaction. When s tems are turned upright after short horizontal stimulation, it takes s ome time for them to recover and adds an erroneous negative movement t o the initial graviresponse curve.