How Does a Venus Flytrap Work?
Digging up the secrets of a plant that senses, moves and digests without nerves, muscles or a stomach
Ferris Jabr • March 14, 2010
The Venus flytrap has mystified biologists since the time of Charles Darwin. Any tiny critter unlucky enough to spring one of the carnivorous plant’s traps will find itself imprisoned in the blink of an eye. But how can a plant — which has no nerves or muscles — move so quickly? It turns out the Venus flytrap is a power plant, capable of generating electrical signals.
Each trap is actually a modified leaf: a hinged midrib, which would be the central vein of a more familiar leaf, joins the two lobes, which secrete a sweet sap to attract insects. The rims of each lobe flair out in a curved row of spikes that interlock when the traps shut to prevent prey from escaping.
When a trap is open, each lobe is convex in shape, its red belly bulging outwards. When something triggers the trap, the lobes flip to a concave arrangement in less than a tenth of a second, forming a sealed stomach. But how does a plant manage such rapid movement?
On the inside of each lobe are three or more tiny sensitive hairs. If an insect, spider or human finger touches more than one of these hairs — or the same hair more than once — within a 30-second window of time, the trap will snap.
[A closeup of a Venus flytrap trigger hair; Credit: Martin Brunner, Wikimedia]
“These hairs are mechanosensors,” said Alexander Volkov, a plant physiologist at Oakwood University in Alabama who has intensively studied how the Venus flytrap works. “The hairs transfer mechanical energy into electrical energy.” The pressure of an insect brushing against the hair is translated into a little electrical charge that travels to the midrib of the trap.
As this electrical charge moves towards the center of the trap, it opens specialized pores in the outermost layer of the trap’s cells, allowing water to rush from cells on the inside of the lobes to cells on the outside. The dramatic change in cell pressure (turgor) flips the lobes, which snap shut.
It might seem odd that a plant relies on electrical signaling, especially since plants don’t have nervous systems. But the truth is that all plants are electrical. In fact, all living cells are electrical. It comes down to some very basic biology, chemistry and physics.
All cells use a membrane to separate what’s inside them from what’s outside them. Only very small molecules can easily enter and exit the cell, but most of these travel through special pores or channels found within the membrane. Some of the most common migrants are called ions — charged particles like sodium, potassium, chloride and calcium.
Whenever a membrane separates different concentrations of ions, it creates the potential for an electrical current. But this electric potential is not left to its own devices. Special protein channels and pumps embedded in the cell membrane act as gatekeepers, regulating the flow of charged particles across the cell membrane. The controlled flow of ions in and out of a cell is what we call electrical signaling — and it happens in both plants and animals.
“In any cell you have a membrane,” explained Volkov. “You have ions on both sides in different concentrations, which creates an electrical potential. It doesn’t matter if it’s an animal or plant cell — it’s general chemistry. Plants have ion channels. In fact, Venus flytrap and you have exactly the same potassium channel.”
So let’s review: insect crawls into trap; insect triggers sensitive hairs; Venus flytrap sends an electrical signal to the center of its trap; the trap snaps shut faster than you can blink your eye.
Having secured its meal, the Venus flytrap begins to eat by releasing an array of digestive enzymes — special proteins that help control the rate of chemical reactions. This acidic concoction dissolves the victim, allowing the plant to absorb the nitrogen it can’t get from the nutrient-poor soil in which it grows. Around ten days later, the trap reopens, revealing a crumbling exoskeleton.
Charles Darwin and Carnivorous Plants
Carnivorous plants enchanted Charles Darwin, who marveled at how they defy all expectations of proper plant etiquette. “This plant, commonly called Venus’ fly-trap,” Darwin wrote in Insectivorous Plants (1875), “is one of the most wonderful in the world.”
In his writings, Darwin noted the incredible speed with which the Venus flytrap shuts its traps. He observed how the plant uses these traps to absorb nourishment from insect prey, leaving only a dried out husk when it reopens its jaws over a week later. By painting little black dots on the traps, Darwin was even able to show how their surface layers contract and expand. But he didn’t fully understand the mechanisms by which the plant hunts and digests.
The Venus flytrap’s Habitat and Anatomy
Native to the coastal bogs and savannas of the Carolinas, Venus flytraps grow slowly and close to the nutrient-poor soil they inhabit. If you kneel next to a wild plant, you will likely see a circular arrangement of four to seven flat green stalks that end in blushing, toothed traps. While the broad, elongated stalks perform photosynthesis — the chemical process plants use to turn water, carbon dioxide and energy from sunlight into sugars and oxygen — the crimson traps wait around for insects, spiders or other critters they can seize for their nitrogen, which the plant needs to continue making vital proteins. (If large enough, the traps can catch small amphibians or rodents.)
The Design of the Traps
When a trap shuts, the long spiky hairs on the rims of the lobes interlock, but leave enough room for tiny insects to get out, so the Venus flytrap does not waste energy digesting an insignificant meal. The traps will open sooner if an insect manages to flee, but will close more tightly if the victim struggles.
Interestingly, rain rarely triggers the traps because the likelihood of a raindrop falling in exactly the same place twice in under 30 seconds is negligible — a good thing for the Venus flytrap, who would otherwise starve every time it rained.
How Electrical Signals Travel in Plants
Plant cells can generate electrical currents — action potentials — just like animal cells. In animals, these currents travel along a nerve cell’s main branch, the axon, which has evolved to efficiently conduct electrical impulses. But plant cells don’t have specialized structures for conducting electrical signals, so how do action potentials travel in plants?
According to Volkov, action potentials in plants may travel through phloem — living tissue that transports sugars and other nutrients through a continuous arrangement of tubular cells.
“Animals have neurons and axons,” Volkov said, “and plants have phloem — but the properties are very similar. In both cases you have a cell membrane. In both cases you have ion channels, which keep the propagation of the action potential constant. There’s a not a big difference.”
“We’ve known about electrical signaling in plants for as long as we’ve known about it in animals,” said Elizabeth Van Volkenburgh, a botanist at the University of Washington. “But in most plants, what those signals are for is an open question.”
In the case of the Venus flytrap, it’s becoming clear just how important electrical signaling can be for a plant. But much more research is needed to better understand the exact mechanisms at work. For many years, Volkov lamented, electrophysiology — the field that studies electrical properties of biological cells — largely ignored the electric aspects of plant life.
“If you open textbook on electrophysiology in the 18th or 19th century, they cover both electrical signals in plants and animals,” Volkov said. “But in the beginning of the 20th century, all money for research in electrophysiology started going to medical investigation of nerves and action potentials in animal science. Many plant physiologists were using very old techniques, because the modern techniques were so expensive. But now we are much better able to measure electrical signals in plants in real time.”
Movement in the Plant Kingdom
Venus flytrap isn’t the only plant capable of rapid movement. Volkov also studies Mimosa pudica — known as the sensitive plant — whose leaves fold up instantly when touched. He’s finding that Mimosa relies on electrical signaling too. There’s also the telegraph plant, whose tiny leaflets constantly swivel to monitor the direction of light in the environment. And some of the Venus flytrap’s carnivorous cousins, the sundews, act like hungry anemones, curling their sticky sweet tentacles around whatever insect blunders into them.
These speedy plants are especially interesting to biologists — and they get our attention because we can actually observe their behavior — but researchers have recognized for decades that all plants move. Most plants simply move too slowly to see in real time.
“We see someone jerk their hands back from a flame and we understand that because we are animals,” explained the University of Washington’s Van Volkenburgh. “We don’t see plants moving or behaving because the time frame that plants use is 10 or 100 or 1000 times longer than what we’re used to.”
Some of the best understood plant movements include phototropism — shoots and leaves bending towards light — and gravitropism — roots growing in the direction of gravity, shoots away from it. These processes rely on hormones that change the rate of cellular growth in plant tissues: if one side of a root or shoot is growing faster than another, it’s going to bend. Volkov thinks electrical signaling may be involved too.
Some young plants’ flowering tops and leaves can track the sun’s movement from East to West — a phenomenon called heliotropism that allows for maximum light exposure. And climbing plants, like vines, respond to touch, changing their growth to curl around the first pole, wall, or branch they contact.
So the Venus flytrap is not really a freak of the plant kingdom. All plants move, all plants are electrical and many plants respond to touch. (Okay, maybe the eating animals thing is a little weird…) The Venus flytrap is actually a paragon of plant abilities, bringing into dramatic focus the surprising sensitivity and sophistication of the plant life all around us.
How does the Venus flytrap keeps track of the number of times its hairs have been triggered?
Great article! I was also wondering about the double-trigger mechanism — what sets that 30-second timer?
Chester and Katie,
Thank you so much for commenting!
I have investigated your questions and I have some answers for you.
Each time something triggers one of sensitive hairs, a tiny electric charge travels to the midrib of the trap. But one of these charges alone is not powerful enough to snap the trap. You need at least two of these charges to reach the center of the trap within a 30-second window of time.
Why? Because the cell membranes in the lobes of the trap are electrically leaky, according to Volkov. He recommends thinking of the membrane as a capacitor that can “store” electric charge. But this stored charge will leak over time. After 30 seconds pass, too much charge has leaked away and the ‘timer’ is reset.
So, what we’re dealing with here is a time-sensitive threshold of electric potential. If within 30 seconds enough hairs are triggered – or one hair is triggered enough times – to reach the threshold, the trap will snap.
I hope that helped! Volkov also sent me some research papers with much more technical and detailed explanations, which I would be happy to send to you if you’re interested. Thanks for reading!
That’s a mutant fly trap in the photograph. Each side usually has 3 trigger hairs. That leaf has 4! I’d love to know how selection maintains 3 rather than 2 or 4 (for example). Great post.
Very detailed article. Does the time gap between the 2 triggerings affect the speed of closure?
I get the general principle of the 30 s resetting “clock” of the venus fly trap. However, Volkov’s published results leave me confused. The clock supposedly resets after 30 s. However, the electrical models published in his papers list the membrane capacitance and resistance as 11nF and 5.3 MOhm, respectively. This gives an RC time constant of approximately 5.5/100 s = 0.055 s. That means the membrane capacitance will discharge very fast compared to the 30 s clock. How are these reconciled? Either I am really missing something or something does not add up.
Well i’m still in high school so I might not know a whole lot but I know in humans the ions of a cell are not all used at one time. An action potential only uses a portion of the available ions. This allows multiple action potentials to be fired without causing too long of a recharge time. This is just a guess but perhaps a similar principle is involved in allowing the RC time constant to reach 30 seconds.
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how do they reproduce? how do they spread their seed
this looks not the same in real life!!! i think that it stupid an to look at that
Insectivorous plants by Darwin. Whether i can get that book from amazon? I want to read the original version
how does the size of the venus flytrap affect its digestive rate?
So that’s how our winners work….
I’m wondering… Will a Venus flytrap trap shut in response to a dead insect or a raindrop?
The Venus Flytrap will not respond to raindrops.
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I have never seen a Venus flytrap in real life! That is so cool you have one in your closarosm! I like that it comes back to life after it dies, I have a bad habit of killing plants. I think I may need one of these because I live in Alabama, USA, and we have a lot of flies here. You know what we do to keep flies away? We hang ziploc baggies full of water over doors and windows that are open. Think a Venus flytrap may look less redneck!! Thanks for the tip!
How does the plant know when to open back up? There is nothing moving at that point, so no electricity could be produced…how else could it sense it is done ‘eating’ and open back up to eat again?
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I agree with/accept the explanations for trigger mechanisms, electrical potentials and all but, what gets me wondering most is how did these plants evolve from everyday photosynthesizing plants to being also meat eaters? This subject involving the evolution of decoys, lures and traps, and mimicry is the most bewildering, yet beautiful, aspect of the ascent of life.