[♪ INTRO] When you hear the term  carnivorous plant, your mind might conjure up a few dramatic images.  Like a Venus fly trap snapping its jaws around its prey, or a small  mammal using a pitcher plant as a toilet.

But in the nearly 100 million years  carnivorous plants have been on Earth, they’ve evolved a bunch of  different tricks for catching prey. And some are a lot more subtle than others.

In fact, some plants have  developed ways to trap their unsuspecting prey…underground. And scientists have found a few types of  underground traps that get the job done. If carnivory works so well above ground, why might a plant want its  traps buried in the soil?

Carnivorous plants of all types tend  to live in environments where the soil can’t provide enough of the nutrients  they need, like nitrogen and phosphorus. So instead, they’ve evolved to  get those nutrients from critters. But critters can serve another  important role in a plant’s life.

They can help the plant reproduce. And you don’t want to  accidentally eat your pollinator! So one advantage of underground traps  is that it puts a lot of space between the structures used for killing, and  the structures needed for making babies.

Take Philcoxia, a genus made up  of three Brazilian plant species described back in 2000, living  in low-nutrient and shallow sand. Philcoxia’s underground traps are made from modified leaves and are pretty unassuming. They’re tiny…only about a millimeter  in size…and produce a sticky goo that stops microscopic prey dead in  its tracks until it’s, well, dead.

Plenty of non-carnivorous plants use  adhesive substances as a kind of defense, so before scientists could call these plants  carnivorous, they needed to figure out if they were actually eating the stuff they catch, including tiny worms called nematodes. In order to be considered a carnivorous  plant, that plant must trap, kill, digest, and absorb nutrients from prey, and then  use those nutrients to help the plant grow. So there was a chance Philcoxia  was catching microorganisms, but not actively digesting them.

It’s not carnivory if you just take  advantage of that decaying animal body leaching nutrients into the soil, even  though it died after getting stuck on you. To untangle this mystery, one  team of scientists fed nematodes a diet laced with nitrogen-15, and  then fed these worms to their plants. Compared to most of the  nitrogen atoms found in nature, nitrogen-15 has an extra neutron,  which makes it a little bit heavier.

Plants can’t really tell the difference, so they’ll use it just as readily  as they will the standard stuff. So the team tracked how fast this nitrogen  started showing up in Philcoxia’s leaves. If it happened really quickly,  like within a few days, that would suggest the plant wasn’t  waiting around for the nematode corpses to decay on their own.

They were actively digesting their food. Since it took less than a day for the  leaves to start incorporating the nitrogen, the evidence suggests Philcoxia  really is carnivorous. Sticky traps like these are  referred to as passive traps, because the plant doesn’t move to trap its prey.

And our next example of underground carnivory: the African genus Genlisea uses  another kind of passive trap. Their so-called eel traps are long, thin tubes with an entrance at one end  and a digestive chamber at the other. Small hairs line the inside of each tube,  and all point away from the entrance.

That makes it hard for the prey to turn  around and go back the way it came. It would be like trying to push your way through a narrow tunnel lined with a  bunch of swords pointing at you. The eel traps look just like a small  root, so at the microscopic level they can blend in with the damp surrounding soil and offer a nice surface for unsuspecting  prey to crawl along.

Like Philcoxia, Genlisea plants prey  on microscopic animals and bacteria. But some underground traps can catch larger prey. In 2022, scientists reported the  discovery of a Nepenthes pitcher plant which can catch insects in  its underground pitchers.

This species is found near the top  of a dry and windy mountain ridge on the island of Borneo, and may have  evolved underground traps because the environmental conditions and food  supply are more consistent down there. Now, this species does produce  some above-ground pitchers much higher up on the plant, but most are below ground. Usually, they take advantage of the  empty space created near tree roots, but occasionally, they move a  bunch of soil aside while growing.

That means they have to be made of  sturdier stuff so they don’t collapse. And to provide that extra support,  these pitcher plants have to invest even more energy into growing them…which they  have to recover in order to survive. This suggests there’s quite  the food supply down there.

Early research suggests that  ants make up most of their diet. Now a tunneling ant falling over a  pitcher’s ledge to its doom may sound pretty dramatic, but our last example uses an  active trap that sucks you in. Literally.

Bladderworts, or Utricularia, are a genus of over 200 species found across the globe. They’re named for their bladder-shaped traps, which are either fully submerged  in water or buried in damp soil. The traps work like a suction  bulb, relying on negative pressure.

The plant invests energy to pump water out of the little bladders and create a  vacuum behind the trap door. When unsuspecting prey brushes  against a trigger hair, the door opens and the prey is sucked right in! The whole thing happens in  just a few milliseconds.

Imagine feeling something brush  up against your leg, you blink, and next thing you know you’re  trapped inside a giant stomach. But the traps aren’t perfect. Sometimes the sensitive door  is triggered by accident, and the plant will have to spend  time and energy resetting itself.

As a silver lining though, that false  alarm might suck in nutrients dissolved in the water, or maybe a wayward microbe  that wouldn’t normally trigger the trap, but has terribly bad timing. There is one type of trap  scientists haven’t seen underground, the active snap traps like  those used by Venus fly traps. But a new discovery could  update this list anytime.

So insects and microbes beware! If you think you can escape the plants hunting you by tunneling underground, think again. And if you, dear viewer, think  you can escape this video before I tell you how  awesome you are, think again.

You are awesome! Just watching this video makes you  awesome, at least to us, because it means you’re thinking about the world complexly  and you’re interested in learning stuff. It also helps us reach more eyeballs and earholes.

So thank you for watching SciShow! And if you wanna help us reach  even more eyeballs and earholes, you can become a patron at  www.patreon.com/SciShow. Thank you for being awesome! [♪ OUTRO]