Circumnutation of Phaseolus vulgaris L. cv. Blanc de Juillet twining shoots was examined at the cellular level. Cell growth was monitored inside the free‐moving part of the shoot as a function of time and position with respect to the terminal bud. Complete elongation of epidermal cells required >7 days. A 60% increase in length of the cells in the bending zone was observed. Scanning and transmission electron microscopy studies showed the elasticity of cell walls, the extent of plasmodesmata and endoplasmic reticulum of epidermal cells. The osmotic potential in epidermal cells of the bending zone displayed significant differences only between the concave and convex sides. The pattern of cell growth in twining shoots is discussed. Circumnutation could be seen as a consequence of periodic turgor changes inducing periodic growth.

The apex of growing stems in twining plants describes a rhythmic movement in space called circumnutation. By the method of orthogonal projections, the position in space of the apex can be determined constantly. The mathematical analysis of data allowed us to determine that for Phaseolus vulgaris L. (cv. Mangetout Blanc de Juillet) grown under constant illumination and temperature (25 degrees C) the period is about 100 min. This movement has been related to rhythmic changes in the osmotic potential of the cells located in the bending zone of the stem for a constant period. These variations are longitudinally and laterally coordinated. Treatment with a solution of LiCl at 7 X 10(-3) M supplied to the root system induces a lengthening of the period. The effect of the treatment is reversed by K+ ions. From these results we deduce that rhythmic changes of the membranes are implicated in the circumnutation movements of twining plants.

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