In 1865, Charles Darwin noted that the spiraling tendrils of the cucumber plant allow it to act in un-plantlike ways. By reaching for and grasping a support, then hoisting it toward the sun, tendrils allow the plant to move. Once tethered, the tendrils coil into a helix, shown in this video. Darwin theorized that the spiral acts like a spring, but noticed that unlike a classic spring, which curls in one direction only, cucumber tendrils wind in opposite directions from the tethered ends, forming a kink in the middle. Now, a team of scientists has figured out just how, and why, the cucumber tendril coils in this fashion. By observing the tendrils' formation and mimicking their structure with mechanical and mathematical models, the researchers found that after the tendril grasps its support, a thin, two-layer ribbon of gelatinous cells shrink on one side as they lose moisture, but not on the other. This asymmetric contraction causes the fiber to wind in opposite directions, creating a spring that may stabilize the plant against large disturbances while permitting it to gently move with small forces, like a gentle breeze. When pulled from either side, the fiber does not unwind like a normal spring, but actually winds tighter. Young, moist tendrils do not tend to overwind, the researchers report online today in Science, but mature, dry tendrils do —helping to explain how delicate tendrils gradually stiffen to support a vine laden with cucumbers.
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