An Unusual Stem Bending Response of Xanthium pensylvanicum to Horizontal Rotation.

When a plant is rotated on a clinostat, with its axis parallel to the ground, the axis may remain straight. According to Sachs (9), the successive stimuli of gravity compensate each other and, therefore, there is no tropic reaction. Czapek (4) on the other hand, felt that under these conditions, there is no perception of the gravity by the plant and consequently no tropic reaction. In connection with an attempt to study the effect of clinostat rotation on photoperiodism, a special clinostat was built with a large wheel upon which a number of plants could be rotated simultaneously, with the plants anchored near the periphery of the wheel and not at the axis of rotation. Instead of remaining straight, the stems were predominantly bent along the circumference of the circle in the direction of rotation. The studies reported here are an attempt to find the causal factors of this uni-directional bending.