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.