Plant Tendril Acts as a Spring

close up of a green spiral cucumber tendril with a blurry background

Biological Strategy

Garden cucumber

Andy Carstens

Image: Sandy Sarsfield / Flickr / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

Search this Function

Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

Search this Living System

Cucumber plant tendrils twist due to an asymmetric contraction of an internal fiber ribbon of specialized cells, causing loops to be added when pulled apart instead of unwinding.

Introduction

Instead of spending the energy to build massive trunks as trees do, vining plants—like ivy climbing an old house—“borrow” height from neighbors, grappling onto them and hauling themselves towards the sky. Cucumber plants, for example, grow tendrils that whip around in circles until they snare something they can scale.

“The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring,” wrote naturalist Charles Darwin.

It’s long been known that this is what the tendrils do. But what hasn’t been understood is how they do it.

The Strategy

Once attached an object, a cucumber tendril coils to create a transition point in the middle, with oppositely handed spirals, or , above and below. This coiling helps to shorten the tendril and lift the plant up towards the attachment point.

Scientists at Harvard University decided to unravel the mystery by pulling apart a tendril. When they stretched the two ends apart, they expected it to unwind and flatten. The plot twist was that pulling the two ends apart actually added twists—the first known example of an “overwinding” spring.

To understand why the tendrils would curl more under tension, the scientists analyzed their structures and found relatively flat ribbons of fiber within. These fiber ribbons are made of two layers of cells that contain lignin, a rigid organic . Furthermore, the inside layer has more lignin than the outside, making it stiffer.

Top image shows an intact cucumber coil, and bottom shows the fiber ribbon extracted from tendril. Both respond to strain by overwinding, i.e. increasing the number of coils.

Image: Raoul Pop / CC BY NC - Creative Commons Attribution + Noncommercial

A cucumber tendril in action.

These results showed that as a ribbon grows, it contracts asymmetrically. In other words, the stiffer inside band shortens relative to the outside, pulling the ribbon (and thus the tendril that surrounds it) into a pigtail or spiral.

The exact mechanism of this asymmetric contraction isn’t known, but they hypothesized that water plays a role. Lignin is hydrophobic (meaning it repels water). As tendrils grow, lignin accumulates at higher amounts within the inside layer of cells. The scientists think that hydrophobic lignin expels more water from the inside cells, deflating and shortening them relative to the outside cells.

They also believe that the differential stiffness of the cell layers enables the helix to twist while resisting bending. To model the two layers of cells within fiber ribbons, they added materials with different rigidities to the inside and out of a silicone spring. When they pulled the two ends of the model spring, it behaved exactly as the actual cucumber tendrils did, overwinding and adding more coils. As a result, they concluded that the higher bending stiffness of cucumber tendrils relative to their twisting stiffness makes them resist flattening while adding turns to compensate for the tension.

However, when enough force is applied, a cucumber tendril will eventually unwind. This second-stage unwinding is actually a stronger response to higher tension as compared to overwinding.

The Potential

The scientists called cucumber tendrils “auto-adaptive” springs because of their two-fold response to tension. Given that springs are like batteries for mechanical energy, smart springs and overwinding springs could help us store energy in new ways.

Applications of this strategy could lead to efficient climbing robots or dampers for stability of damaged structures. They could be used to alter surface area for selective collection of oils, dyes, pollutants, or even harmful micro-organisms.

Last Updated July 26, 2023

References

“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).

Journal article