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SciShow describes the fascinating science of Darwin's little darlings: meat-eating plants. Learn about their many different types, how they catch and eat their prey, and how scientists think they evolved.
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Charles Darwin studied thousands of plants and animals from all over the world throughout his career, but it was a plant he encountered in his native England in 1860 that inspired him to write: "I care more about Drosera than the origin of all the species in the world." Any guesses as to what plant that might be?

[intro music]

 The what

Darwin's fascination with Drosera, a kind of plant known as a 'sundew', stemmed from its ability to capture and digest insects. He categorized it, and other plants like it, as 'insectivorous plants'. We know them today as 'carnivorous plants' because, well, they're not that picky. Several species of the plants have been known to trap and digest frogs and even small mammals.

You've probably heard of the Venus fly trap, but did you know that it's just one of more than six hundred known carnivorous plant species, with more discovered every year? Scientists generally look for two things when defining a carnivorous plant: it has to be able to absorb nutrients from a dead animal, and it must have some adaptation that it uses to attract, capture, kill, and digest its prey. But where do these adaptations come from? And why would a plant need to eat meat, when it gets its energy from the sun for free?

 The why

Well, while most plants get their nitrogen and nutrients from soil through their roots, carnivorous plants are typically found in swampy environments like bogs, where water is constantly washing those nutrients away. So they get their nitrogen from animal tissue, absorbed through glands in their specially modified leaves. How exactly they do this varies widely among hundreds of species in at least nine plant families.

 The how

There are pitcher plants, for example, which lure their prey with sweet nectar into leaves that resemble a long tube. Insects fall from the slippery rim of the pitcher into what's known as a 'pitfall trap'. This is filled with a mix of rain water, digestive enzymes, and the leftovers of previous prey. Not exactly a fun way to die...

Then there are bladderworts, which, with over two hundred species, make up the largest group of carnivorous plants. They use bladder-shaped leaves lined trigger hairs and topped with a sort of trap door. When an insect touches one of the hairs the door opens and sucks in the victim. Within fifteen minutes the prey is digested. Quite efficient.

Species of the sundews described by Darwin act much like a spider web, luring and catching insects with sticky drops disguised as nectar. But those drops contain a thick, mucus-like substance that traps the prey on the leaves' sticky tentacles.

And the venus fly trap is well known for a good reason. Its eponymous trap activates when an insect walks across the leaf and applies pressure to its trigger hairs, but it doesn't initially close all the way. Scientists believe that this is the plant's way of letting smaller bugs escape, so it doesn't waste time digesting a low-nutrient meal. Instead, it closes a second time soon after, using enzymes similar to those in our stomachs to slowly digest its prey. Unlike other carnivorous plants the venus fly trap can take up to ten days to finish its meal.

 The origin

For more than 150 years carnivorous plants in their astounding diversity have fascinated and perplexed botanists, and until the late 1980s many scientists thought they all shared a common ancestor. But studies in the last twenty five years have shown that carnivory, as it's called, evolved independently at least six times within five orders of plants. Carnivorous plants are a pretty wonderful example of convergent evolution, in which unrelated organisms develop similar traits in response to their environment, in this case nutrient-poor swamps and bogs all around the world.

 Closing notes

Thanks for watching this episode of SciShow, brought to you in part by Audible, which is giving a way a free audiobook to SciShow viewers. Head on over to Audiblepodcast, where you can download Charles Darwin's On the Origin of Species, or practically any other book you could want, and listen to it for free. That's Audiblepodcast! And remember, if you have any questions or comments, you can find us on Facebook and Twitter, or in the comments below. And if you want to keep getting smarter with us, go to Youtube and subscribe.