Chara rhizoids, cylindrical cells growing in tbeir apex, having the capability of positive orthogravitropic response, while growing without any change of position arc not idcally straight, but they arc characterised by slight and repeating cyclic bendings. The mean distancc between successive bendings is 150 µm. The bendings appcar morc or less every 1.5 hour at rhizoids' growth rate of abo ut I 00 µm h-1• Aftcr the displacement of the rhizoids from the vertical to the horizontal position, during the g, ,vireaction, the curvature proceeds in stages, in which, alternatingly, there appear 1,;riods of higher or !ower curvature. The curvaturc of succcssivc higber bendings decreacs when the rhizoid reachcs the vertical direction. It seems that gravireaction is based on the increase of the maxima) curvature of cyclic bendings and directing the bendings into a vertical piane. Other tip growing cells, generally straight, such as root hairs or hyphae of Mucor and Phycomyces, also show the repeating bendings
