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"The helical coiling of plant tendrils has fascinated scientists for centuries, yet the underlying mechanism remains elusive. Moreover, despite Darwin’s widely accepted interpretation of coiled tendrils as soft springs, their mechanical behavior remains unknown. Our experiments on cucumber tendrils demonstrate that tendril coiling occurs via asymmetric contraction of an internal fiber ribbon of specialized cells. Under tension, both extracted fiber ribbons and old tendrils exhibit twistless overwinding rather than unwinding [that is, instead of unwinding to a flat ribbon under stress, as an untwisted coil normally would, the cucumber’s tendrils actually coil further], with an initially soft response followed by strong strain-stiffening at large extensions [i.e., as the strain on the tendril increases instead of the coils unravelling as might be expected the number of coils increases]. We explain this behavior using physical models of prestrained rubber strips, geometric arguments, and mathematical models of elastic filaments. Collectively, our study illuminates the origin of tendril coiling, quantifies Darwin’s original proposal, and suggests designs for biomimetic twistless springs with tunable mechanical responses." (Gerbode 2012:1087).
How the Cucumber Tendril Coils and OverwindsScienceAugust 30, 2012