What we do
We want to understand how organisms deal with physical problems in their environment, and find mechanical solutions to ecological challenges. A lot of our work focuses on plants and their interactions with other organisms and with the weather. How do carnivorous plants trap insects? How can leaves withstand the impacts of hailstones? How do plants control wetting and water spreading on their surfaces? Why are some plant surfaces slippery for insects? How do plants make these surfaces? We tackle these and other questions with a combination of lab-based biomechanical experiments and experimental ecological field work. This ensures that we get the best of both worlds: quantitative accuracy and controllability in the lab, and functional and ecological relevance in the field. In addition, we work closely with theoretical modellers, biochemists, engineers and phylogeneticists to help us understand how plants have adapted their surfaces and biomaterials to rise to the diverse challenges they face in their environment.
Whisk-like hairs keep the floating leaves of Salvinia molesta dry even when they are submerged. While most of the whisk is water-repellent, the central tip is hydrophilic. As a result, water droplets like the one above are ‘pinned’ in place and held way away from the underlying leaf surface. (Category winner Evolutionary Biology in the inaugural Royal Society Publishing Photography competition in 2015.)
Why physics matters
In contrast to the chemical side of ecology, mechanical influence factors and adaptations are poorly understood. But plants (and all other organisms) live in a physical world, and plants in particular cannot run away and seek shelter when a storm rolls in or an army of hungry insects descends on them. Damage to crops from hail and wind is an important economic factor, and with a growing world population and increasing frequency of severe weather events due to climate change, we need to find sustainable solutions to making our crops more resistant to physical damage. We want to understand how plants respond to physical selection factors, and how they can adapt their support structures and surface properties to help them negotiate the challenges of their physical environment.
Which plants we look at
We work a lot with carnivorous plants, in particular pitcher plants. This is not only because they are really cool plants that trap and kill insects in specialised cup-shaped leaves, but also because they are an excellent model system to look at mechanical ecology. Many plants have modified their surfaces to repel insects, but the slippery trapping surfaces of pitcher plants are unparalleled in effectiveness and diversity of underlying physical mechanisms. Minute wax crystals cover some of the surfaces and create fine-scale surface roughness that reduces the available contact area for insects’ sticky footpads. To see how it works, try sticking some sellotape to fine-grain sandpaper! Other surfaces are unusually wettable so that rain or dew covers them in a film of water that makes insect feet slip like car tyres on a wet road.
The collar-shaped rim of Nepenthes pitcher traps is safe for insects to walk on when dry, but turns extremely slippery when wet. This environmental ‘switch’ makes traps unpredictable and ensures that ‘scout’ ants can safely harvest nectar during dry times, thereby promoting ant recruitment to the trap and, ultimately, increased prey intake.
The bottom of the trap is filled with a digestive liquid, and again, the plants use mechanical mechanisms to their advantage: in many species, the liquid contains polysaccharide (sugar) macromolecules that make it strongly viscoelastic. You can imagine the polysaccharides like a three-dimensional spider web that is made of highly extensional coils of string. When tension is applied to this web in any direction, it can stretch and deform easily without breaking. So when you pour out the fluid from a pitcher, it forms very long – up to one metre! – coherent threads that will even rebound and flow back up if you tip the pitcher back upright before the thread has become too long. When an insect falls into the fluid, it literally wraps itself in these fluid threads, leaving little chance of escape.
Nepenthes pervillei, one of the phylogenetically oldest species of pitcher plant and endemic to two tiny islands in the middle of the Indian Ocean, growing high on the slopes of Mont Pot à Eau on Silhouette island, Seychelles.
- Adaptations of plant leaves for avoiding, dissipating or exploiting impact energy (PhD project: Anne-Kristin Lenz)
- The effect of plant surface micro-topography on wetting, surface contamination, and insect adhesion and locomotion (PhD project: Michal R Golos)
- Developmental biology of superhydrophilic, directional, antiadhesive plant surfaces (MRes project: Oonah Lessware)
- Development, plasticity and evolution of slippery wax crystal surfaces in Asian Nepenthes pitcher plants (in collaboration with Lucas Busta, University of Minnesota Duluth, and Reinhard Jetter, University of British Columbia)
- Viscoelastic fluids for prey capture – properties and function (in collaboration with Walter Federle, University of Cambridge)
- Ants and pitcher plants – exploitation or mutualism?