Imagine a spring that is flexible when pulled gently and more stiff under greater tension. Using principles of physics executed with biological tools, nature has devised a way to do this to the benefit of vining plants everywhere. Unlike a coil that rolls out flat when pulled at each end, cucumber tendrils overwind at first. Curiosity motivated researchers at Harvard University to build silicone models to try to explain the phenomenon. One characteristic of the tendril structure observed by many (including Charles Darwin) is a change in helical direction midway along the tendril. Special threadlike, gelatinous fiber cells running along the tendril were recently discovered. The key to the behavior of this two-cell-thick ribbon lies in the building materials. The inner cell layer has more lignin than cellulose; lignin is like packing material between the rebar-like cellulose. Also, as the tendril matures, water molecules are excluded from the tendril, causing it to shrink. The differing ratio of building materials in the two cell layers of the ribbon, and the shrinkage during maturation, causes coiling.
Science Vol. 337 no. 6098 pp. 1087-1091