Catapult Plant Flings Prey into Botanical Maw

September 22, 2012 § Leave a comment

Picture yourself in the place of one of southern Australia’s many walking insects—say, a springtail or an ant. You’re making your way along the urbanized bushland of Adelaide, when out of nowhere, you’re thrown into the air, zipping up and over into a sticky nest of plant tentacles that promptly stir to life and begin to inexorably stuff you into a botanical pit of gastric juices.

Congratulations! You have met Drosera glanduligera, the tiny, carnivorous Australian Pimpernel Sundew and the world’s only known prey-catapulting plant. It’s a relatively recent arrival to the science lore and despite the unusual skill set it brings to the table, researchers actually know remarkably little about it.

See, most sundews operate very simply: insects attracted to a plant’s dew-like glue beads become ensnared, suffocate, and are digested in a soup of enzymatic juices. To minimize escape and maximize contact, most sundews curl their sticky tentacles around their prey, moving at pace of seconds to hours, depending on the species. However, in no world does any sundew move in fractions of a second—them’s Venus Flytrap speeds, you could say (if you wanted to say it like that), with no rightful place in the Drosera genus.

Such, at least, was the belief maintained by scientists until 2003, when the Pimpernel’s lone champion, a carnivorous plant enthusiast named Richard Davion, mailed proof of his backyard sundews in action to German botanists Irmgard and Siegfried Hartmeyer, who’ve researched the species ever since (and in fact, coined the term ‘snap-tentacle).

This week, a new collaboration between the Hartmeyers and plant biomechanics researchers from the University of Freiburg, Germany, appears in the journal PLoS ONE. The study provides the first formal documentation and analysis of D. glanduligera‘s tentacles in action, as analyzed in response to live prey, under a microscope while stimulated with nylon thread, and in still images captured under a scanning electron microscope.

The way it works is this: each Pimpernel maintains numerous inner glue-tentacles, located around a central depression. Rimming the outside of the plant are 12 to 18 snap-tentacles (schnelltentakel) that are a little longer, glue-free, and completely unique to this species. When an insect steps onto the touch-sensitive tip of the snap-tentacle, the catapult system kicks into gear, snapping at a specific hinge, and tossing the bug towards the middle of the plant, all in as few at 75 milliseconds—speeds more than four times the blink of an eye. The touch of the prey on the sticky inner tentacles in turn sends those organs into gear and they begin to slowly roll the hapless insect towards the central basin of the plant where digestion occurs (see: harrowing introduction above).

There are two potential models that could explain the schnelltentakel‘s mechanics. First, hydraulics: water rushing from the swollen top layer of the tentacle’s hinge and into the bottom could force a sudden pressure change, resulting in a contraction. Alternatively, tension differences between the top and the bottom layers of the hinge could pre-store energy, holding the tentacle at the ready (this is essentially how the sundew’s cousin, the Venus flytrap, snaps shut).

Now, the electron microscopy did find that the cells lining the bottom-most layer of the hinge are about half the size of those on the top, which could store energy by way a sort of elastic instability. However, unlike the flytrap, the sundew can move at faster and slower speeds depending on conditions like temperature and humidity, which points strongly towards hydraulics. “If there were a role of elastic instability playing a role in this movement, it could not be slow or fast.” says Simon Poppinga, a researcher with the Plant Biomechanics Group and lead author of the paper. “It would always be the same. It’s like a bow-and-arrow.”

The answers to these questions go beyond scientific curiosity (although in fairness, curiosity is a pretty big part of it). The Plant Biomechanics Group that led this work does not devote itself to basic research only. In fact, it specializes in translating natural forms and functions into technology: going from the tough skin of the pomelo fruit to crash protection technology, developing self-repairing foams based on liana vines, that sort of thing. In fact, it was Poppinga’s own research in Bird-of-Paradise flowers, that resulted in the flexible architecture debuted this summer in the Soma Theme Pavilion at Expo 2012. While so many basic electrophysiological questions remain, that type of research is a ways away for the Pimpernel sundew, but it’s not out of the question.

“What we think is interesting in plant movement like this is that they work with a complete absence of technical hinges,” Poppinga says—there’s nothing to break, stick, or demand a fresh coat of oil. Certainly, elements of the Venus flytrap’s spring-loaded snap have already made it into modern technology… what could we do with catapulting tentacles?

Reference & photo credit: Poppinga S, et al. Catapulting Tentacles in a Sticky Carnivorous Plant. PLoS ONE, 2012, 7(9):e45735. doi:10.1371/journal.pone.0045735.

Bonus vid for your edification and entertainment:

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