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, with an initially soft response followed by strong
strain-stiffening at large extensions. 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.