| Summary: | Chara rhizoids, cylindrical cells growing in their apex, having the capability of positive orthogravitropic response, while growing without any change of position are not ideally straight, but they are characterised by slight and repeating cyclic bendings. The mean distance between successive bendings is 150 µm. The bendings appear more or less every 1.5 hour at rhizoids' growth rate of about 100 µm h-1. After the displacement of the rhizoids from the vertical to the horizontal position, during the gravireaction, the curvature proceeds in stages, in which, alternatingly, there appear periods of higher or lower curvature. The curvature of successive higher bendings decreases when the rhizoid reaches the vertical direction. It seems that gravireaction is based on the increase of the maximal curvature of cyclic bendings and directing the bendings into a vertical plane. Other tip growing cells, generally straight, such as root hairs or hyphae of Mucor and Phycomyces, also show the repeating bendings.
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