Pitcher plants are all wet

Plant rim's fully wettable surface is major trap mechanism missed by fairweather scientists

Cathy Holding(cathy.holding@absw.org.uk)
Sep 20, 2004

Pitcher plants' primary way of catching prey may have been missed for more than a century because scientists didn't like working in the rain in rainforests, according to Walter Federle, coauthor of a study published in the online edition of PNAS this week.

Until now, researchers believed that pitcher plants—studied since the 17th century, Federle said—captured most insects using waxy crystals making slippery inner walls. However, when Federle's team studied Nepenthes bicalcarata—one of the few species of pitcher plants that have no slippery waxy inner walls but are nevertheless able to capture insects—after a rainfall, they found "lots of ants just falling one after the other into these pitchers," Federle told The Scientist. That was remarkable, he said, because seeing even single insects trapped is usually a very rare event.

The researchers found that the rim of the pitcher plant—one of several carnivorous plants that traps and digests insects as a primary nitrogen source—has a microstructure that makes it a fully wettable surface when covered with rain, dew, or nectar, but never slippery when dry. Insects "aquaplane" into the pitcher's waiting digestive juices without their various feet touching the floor, but the rim, or peristome, has to be wetted first—and so did researchers looking for it.

The group, based at the Department of Zoology II, Biozentrum, in Germany, later found the same fully wettable surface on the peristomes in other species in that genus—even in those with waxy walls—and found that the capture rate in these other species is three times greater when wet than when dry. "So it means it's probably the most important general mechanism for how these pitcher plants capture insects," Federle said.

Because peristome wetting is entirely unpredictable, Federle said, he believes it is a good strategy that lets the plant capture large numbers of ants. The dry peristome allows the ants to reach the nectar without capturing them. "You have to make sure that you are not capturing the scout—the first ant that goes to the pitcher—you have to make sure that it goes back to the nest to recruit nest mates, so the ants arrive in large numbers," Federle explained. Intermittent wetting allows the plant to capture many ants at one time before drying out again, allowing large-scale trapping of prey at the same time that recruitment is going on, he said.

The absence of the waxy inner surface of this species of pitcher could be due to a remarkable mutualism the plant shares with a particular ant, Camponotus schmitzi, which not only harvests nectar but also goes into the pitchers themselves to retrieve captured prey, according to Federle.

"It's an amazing evolutionary story," said Reinhard Jetter, assistant professor in the departments of botany and chemistry, University of British Columbia. "This species of ant is adapted to walk on these slippery surfaces and get prey from the plant." The ants are able to get in to the pitcher, swim and dive in the pool, and then to get back out again, while all other insects were trapped and drowned within 5 minutes. "They can somehow overcome all those barriers, even the complete wettability," said Jetter, who was not involved in the study.

"It would be really interesting to see if this same sort of phenomenon works in the other two pitcher plant families, the Cephalotaceae and the Sarraceniaceae, because they're not in the same plant orders," Aaron M. Ellison, senior ecologist and senior research fellow at Harvard University, told The Scientist.

Carnivorous plants in general are one of the best examples of convergent evolution, and the three families of pitcher plants are completely unrelated to each other, said Ellison, who was not involved in the study. "It would be really interesting to know if there are similar mechanisms of prey capture going on in the other two families," he said.