3 Gangsters of the Plant Kingdom

Most plants are sweet, sunlight-chugging citizens. These carnivorous kingpins are not.

1. Nepenthes bicalcarata

You know a pitcher plant when you see one. Marked by vicious-looking “pitfall traps” rimmed by a slippery surface and filled with a fluid that drowns and dissolves insects, the plants are unmistakable killers. In fact, their fearsome physiques make pitcher plants the poster children for carnivorous flora. But one pitcher seems to be the black sheep of the genus. The Nepenthes bicalcarata lacks the slippery walls needed to capture and contain prey, and its digestive fluids aren’t nearly as acidic as its cousins’.

While that may seem like a disadvantage, the plant gets plenty of help from its friends. Small groups of Camponotus schmitzi ants reside in the swollen tendrils at the base of the plant’s pitcher. In exchange for room and board (nectar secreted from the pitcher rim and a few bites of anything caught), the ants roll up their sleeves and get to work. The colony’s main duty is to fend off weevils that would otherwise eat developing pitcher buds. Additionally, the ants are great housekeepers, cleaning the mouth of the pitcher to keep the entrance as slippery as possible. Science shows that ant-colonized plants benefit significantly from the collaboration: They produce bigger leaves and larger pitchers. Everyone wins—except the prey.

2. GREEN BEANS

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When a Canadian study revealed that pine trees were cutting nefarious deals with a local fungus, using it to kill insects and harvest nutrients in exchange for carbon, researchers wondered whether other plants were making similar pacts. To test the theory, the scientists set up a devious experiment using the genus Metarhizium, killer fungi that infect more than 200 species of insects by eating the bugs from the inside out. They started by burying a tight mesh screen. On one side, they placed insect larvae loaded with both a Metarhizium fungus and an uncommon isotope of nitrogen, N-15, which isn’t found in the soil. On the other, they planted a seemingly harmless plant—green beans. The screen prevented the larvae and plant roots from interacting or encroaching on the other’s side.

Two weeks later, all the larvae were dead, and N-15 accounted for a quarter to a third of the nitrogen in the beans. There was only one way the plants could have gotten the nitrogen: The fungus had killed the insects and transferred the nutrients to the plants on the other side of the screen. Since Metarhizium fungi live in and around plant roots all over the world, it’s likely that the cold-blooded green beans aren’t the only plants secretly using the fungal hit men to supply them with meat.

3. Roridula gorgonias

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When Charles Darwin first encountered Roridula gorgonias, he knew he was staring at a predator. A dried sample of the plant had turned up in Darwin’s lab for analysis, and while he was certain it ate insects, he wasn’t sure how. Over the next hundred years, scientists debated the verdict, finding that the plant can catch bugs but not eat them. The leaves of the South African shrub are covered in sticky hairs that efficiently trap insects of all sizes, but the plant has no obvious mechanism to consume what it catches. Furthermore, it doesn’t have any of the glands for producing digestive enzymes or sucking up nutrients from dead bugs. So what was it doing with the catch?

It took more than a century of scientific back-and-forth, but in 1996, South African researchers finally proved that R. gorgonias was a flesh eater (at least indirectly) by shaking down its partner in crime, Panerudea roridulae.

The tiny bug lives exclusively in the leaves of R. gorgonias, and it’s built to thrive in that environment. Its body is covered in a grease that keeps it from getting stuck in the plant’s resin, and it feeds on any insects that get trapped. While this carrion bug seems to be stealing the plant’s captives, it’s actually sharing the feast by acting as an external stomach. After devouring the trapped insects, the P. roridulae excretes onto those same leaves. With all the digestive heavy lifting completed, the plant can absorb some predigested nitrogen and other nutrients. Seems like a fair trade for putting up with the bug’s crap!

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