PhD project: Anne-Kristin Lenz
Adaptations of leaf material properties and geometry may function to dissipate impact energy or result in spring-like properties that enable the plant to utilise the impact energy to dislodge insects or contaminating particles. For example, we are investigating the biomechanical adaptations that enable carnivorous Nepenthes gracilis pitcher plants to exploit the impacts of rain drops to capture insect prey. The roof-like ‘lid’ of the cup-shaped trapping leaf of this plant functions as rain-actuated spring that literally catapults sheltering insects from its underside into the deadly trap below.
The spectacular lid trapping mechanism of Nepenthes gracilis can only be fully appreciated when viewed in super-slow motion (filmed here at 2,000 frames per second). In reality, it is over in the blink of an eye.
Anne used a high-resolution CT scanner to visualise the detailed deformation of the pitcher during an impact-induced lid oscillation. Surprisingly, she found not one, but two deformation ‘hotspots’: one in the narrow constriction between the lid and the pitcher body, and a second one much lower down, in the rear ‘spine’ of the pitcher body. In other words, the Nepenthes gracilis ‘springboard’ trap is powered by a stacked dual spring. Even more extraordinarily, this spring is much stiffer when the lid moves upward, compared to when it moves down. By repeating her experiment with a 3D-printed pitcher replica, Anne could show that the spring function is purely based on the geometry of the pitcher. In other words, the spring regions that Anne identified in her earlier experiments do not need to specialised material properties – a replica with uniform material properties behaves qualitatively in the same way as the natural plant trap!
A 3D-printed replica (1.5 times natural size) deforms qualitatively very similar to its natural template . (A) Longitudinal section through the dorsal ‘spine’ of the artificial replica (black, left side) and the real pitcher (green, right side). Measurement points for B and C (indices) are marked with crosses and numbered. The blue line shows the deformation at the highest point of the lid oscillation, and the yellow line the corresponding deformation at the lowest point. The middles (grey) line shows the neutral position of the lid when it is not moving. Panel (B) shows the curvature of the dorsal ‘spine’ in each of the measurement points (again, print on the left, plant on the right), and (C) shows the change in curvature between the neutral and the highest lid position (in blue), and between the neutral and the lowest lid position (in yellow). This panel in particular shows that the replica and the real pitcher respond in nearly identical ways to the change in lid position.
Understanding the properties underpinning the ‘lid spring’ or the remarkable impact resistance of some leaves may inspire novel approaches to energy harvesting from natural impact forces, or engineering solutions for improving the impact resistance of man-made buildings.