The cup-shaped leaves of Nepenthes gracilis, or the slender pitcher plant, are equipped with a canopy-like hanging lid that turns into a ‘springboard’ for ants when it is hit by a falling raindrop. New research from the University of Bristol reveals how the spring works.
Nepenthes gracilis is a species of tropical pitcher plant in the monotypic family Nepenthaceae.
This insect-eating plant is widespread in Borneo, Sulawesi, Sumatra, Cambodia, Peninsular Malaysia, Singapore, and Thailand.
“Carnivorous pitcher plants capture insects in cup-shaped leaves that function as motionless pitfall traps,” said Anne-Kristin Lenz and Dr. Ulrike Bauer from the School of Biological Sciences, University of Bristol.
“Nepenthes gracilis evolved a unique ‘springboard’ trapping mechanism that exploits the impact energy of falling raindrops to actuate a fast pivoting motion of the canopy-like pitcher lid.”
Lenz and Dr. Bauer were surprised to find that, rather than bending in the lid itself or in the narrow constriction between pitcher cup and lid, the spring is located far down in the back of the tubular pitcher wall.
The off-center location at the rear of the tube has two effects.
First, it makes the spring direction-dependent and as a result, the lid moves easily down, but not up.
When a rain drop hits, the lid is accelerated quickly downwards, flicking any insects sitting on its underside into the fluid-filled trap below.
On the way up though, the increased resistance of the spring slows the lid down, so that it stops moving sooner and the trap is quickly ready to capture again.
Second, the off-center spring prevents the lid from twisting or wobbling, thereby maximizing the transmission of impact energy into downward movement.
“If you look at the pitcher shape you would assume that the deformation happens at the smallest cross section, which is the transition point from lid to pitcher tube, but in fact it also deforms further down at the back of the pitcher tube,” Lenz said.
“Pitcher plant traps are lightweight, but sturdy. Nepenthes gracilis uses small changes in the trap shape to transmit impact energy with astounding efficiency.”
“We can learn from these plants how to optimize structures geometrically, which could help to save material and weight, while still having a functional spring.”
“The springboard trapping mechanism might even provide inspiration for designing new mechanical devices for harvesting energy from rain or hail.”
“This clever use of geometry makes Nepenthes gracilis the only known plant to exploit an external energy source to achieve extremely fast movement — entirely free of metabolic costs.”
The findings are published in the journal Biology Letters.
Anne-Kristin Lenz & Ulrike Bauer. Pitcher geometry facilitates extrinsically powered ‘springboard trapping’ in carnivorous Nepenthes gracilis pitcher plants. Biology Letters 18 (8): 20220106; doi: 10.1098/rsbl.2022.0106
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