May 18, 2018 | David F. Coppedge

This Plant Beats the Venus Flytrap for Speed

The waterwheel plant, an aquatic species with a snap-trap faster than the Venus flytrap, is different enough to challenge evolutionary speculations.

Most people have heard of or seen the Venus flytrap, one of nature’s most remarkable plants. In a split second, its leaves snap shut around any insect unlucky enough to touch its trigger hairs twice. But have you heard of the Waterwheel plant? Look at its picture in Science Daily reproduced from a press release at the University of Freiburg. Smaller than the Venus flytrap, with traps only 3mm in length, it boasts a much higher snapping action—ten times faster than the Venus flytrap. It was named the Waterwheel because its spokes stick out from the center with a trap on each end. From a water flea’s perspective, it should be called the Ferris Wheel from Hell.

An article on the BBC News shows how the waterwheels are arranged along a stem, and says that the plants, though rare, are native to Europe, Africa, Asia and Australia. Being the only other known plant with snap traps, is it related to the Venus flytrap by evolution?

Credit: Plant Biomechanics Group.

The Venus flytrap (Dionaea muscipula) and the far less known aquatic waterwheel are the only carnivorous plants with snap traps. While intensive research on the Venus flytrap has been going on for a long time, the ten times faster underwater snap traps of the waterwheel have so far been little studied. The team led by the Freiburg biologists has now deciphered the underlying movement principle using experiments and computer simulations. The researchers found that the waterwheel snaps shut its trap, which is only three millimetres in size, by actively changing the internal pressure in the cells of the leaf, which leads to the midrib bending, and also by releasing internal prestress, which apparently results in an acceleration effect. The Venus flytrap, on the other hand, employs a hydraulic mechanism to change the curvature of its leaf halves which results in rapid trap closure. Although both plants share many similarities, the mechanics of the traps differ considerably. This finding may not only help understanding the development of snap traps from an evolutionary perspective, but also the adaptation to different habitats – in a terrestrial habitat with the Venus flytrap, under water with the waterwheel.

The Waterwheel plant was the subject of two scientific papers this month. The first one, by Westermeier et al. in the Proceedings B of the Royal Society, reports on “How the carnivorous waterwheel plant (Aldrovanda vesiculosa) snaps.” The authors claim that their study of the mechanical trap of the Waterwheel plant “contributes to the question of how snap-traps may have evolved” but, since the paper is behind a paywall, we cannot analyze their arguments here. It appears from the press release, though, that there are significant differences between the two snap-trap species: differences in—

  • Habitat, one terrestrial and one aquatic
  • Size, one hand-size and the other made of trailing stems
  • Growth habit, one upright and the other trailing
  • Mechanics, one employing hydraulics and the other internal pressure in the cells
  • Speed, the Waterwheel being ten times faster

There is no evidence the two plants are related. “It is thought that the waterwheel and the flytrap may share a common ancestor,” Mary Halton says in the BBC News article. “However there is no fossil evidence for what this ancestor might have looked like.”

“This is one of the main questions in the carnivorous plant community,” says Dr Simon Poppinga, an author on the study.

Snap traps evolved only once in plants. There are two different mechanisms. Which one was first?

The second paper focuses on the design of the Waterwheel plant to come up with a biomimetics application: “Flectofold—a biomimetic compliant shading device for complex free form facades,” by Körner et al. in Smart Materials and Structures

Illustration from Curtis’s Venus flytrap, drawing from Botanical Magazine by William Curtis (1746–1799)

Smart and adaptive outer façade shading systems are of high interest in modern architecture. For long lasting and reliable systems, the abandonment of hinges which often fail due to mechanical wear during repetitive use is of particular importance. Drawing inspiration from the hinge-less motion of the underwater snap-trap of the carnivorous waterwheel plant (Aldrovanda vesiculosa), the compliant façade shading device Flectofold was developed. Based on computational simulations of the biological role-model’s elastic and reversible motion, the actuation principle of the plant can be identified. The enclosed geometric motion principle is abstracted into a simplified curved-line folding geometry with distinct flexible hinge-zones. The kinematic behaviour is translated into a quantitative kinetic model, using finite element simulation which allows the detailed analyses of the influence of geometric parameters such as curved-fold line radius and various pneumatically driven actuation principles on the motion behaviour, stress concentrations within the hinge-zones, and actuation forces. The information regarding geometric relations and material gradients gained from those computational models are then used to develop novel material combinations for glass fibre reinforced plastics which enabled the fabrication of physical prototypes of the compliant façade shading device Flectofold.

Readers may wish to score these two papers on their scientific value. The first one does produce original knowledge on the plant’s mechanics, but ends with a just-so story about evolution. The second paper yields a practical application that could improve our lives.

Darwinian storytelling is so useless. Give it up, researchers! We don’t need an “evolutionary perspective” on everything. Give us the facts, and make something useful from what you learn to help your fellow human beings.

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