Why a Venus Flytrap Waits for a Second Touch
The surface of a Venus flytrap leaf has a few tiny trigger hairs. Researchers at the University of Würzburg in Germany report that one touch is not enough to close the trap. A single signal could be a false alarm, such as a raindrop or a bit of dust. The second touch makes the trap snap shut in the blink of an eye [1].
If the trapped insect keeps struggling, it keeps brushing the hairs. The same team measured about 60 signals per hour from a trapped insect. When they nudged single hairs by hand to imitate this, two or more touches switched on the pathway of a touch-and-wound hormone called jasmonate. At five or more touches, the plant also switched on the genes for digestive enzymes across its roughly 37,000 glands [1].
So the plant is counting, in a sense: it gives a different answer at one, two, and five. The big question is what does the counting when there is no brain to do it.
What Electrical and Calcium Signals Do Inside a Plant
Each touch on a trigger hair makes the flytrap fire an electrical impulse called an action potential, which spreads across the whole trap. The Würzburg team's news release describes the impulse as linked to a calcium signal and a calcium wave [2].
An action potential is a quick, short-lived change in the electric charge across a cell's outer membrane. It is "all-or-nothing": it either happens fully or not at all [4]. Calcium works in a different way. A cell normally keeps its calcium very low, so when channels open and a little calcium rushes in, the change is large and easy for the cell to read as a message. (The idea that calcium inside cells is kept much lower than outside comes from the Saitama University release listed in the sources [3].)
The two signals are linked rather than separate. One review says the electrical and calcium signals can depend on each other as they travel, and that this can lead to the hormone jasmonate being made in distant tissue [4]. The same review describes electrical signaling as the most conserved type of systemic signaling in eukaryotes (the group that includes plants and animals) [4]. The plant has no nerves, but it shares some of the same basic tool kit as animals.
How a Plant Without a Brain Counts Touches
The Würzburg team's explanation is that the count is stored in the calcium signal. "Counting is about measuring the individual calcium spikes," said biophysicist Rainer Hedrich, who led the work [2]. Each touch adds one spike, and the trap's later steps respond to how many spikes have piled up. Hedrich's group also reported that the flytrap can remember the number of touches for at least four hours [1].
A strange plant gave a clue. At a plant fair, a team member found a flytrap that did not close on the second touch and did not start digesting after more touches. Hedrich named the mutant DYSC, short for dyscalculia, a difficulty with numbers. When the researchers measured it, the touch sensing and the electrical impulse looked normal. Adding jasmonate fixed the digestion step but not the fast closing. They concluded that the fault is in decoding the calcium signal, which is the step after the signal is made [2].
That is a good scientific lesson: "counting" here does not mean the plant thinks about numbers. It means the strength of the response depends on how many signals arrive. Exactly which molecules read the calcium spikes was still being worked out when the report was written, and the team said it was following two lines of research to find them [2].
How a Chewed Leaf Warns the Rest of the Plant
Flytraps are unusual. What about an ordinary plant being eaten? A team at Saitama University in Japan studied a small flowering plant called thale cress (Arabidopsis thaliana) and made calcium visible with a glowing sensor. When a caterpillar chewed a leaf, calcium rose in that leaf within about 20 seconds, and in the study described by Saitama University the signal reached younger, unharmed leaves at 40 to 60 seconds. The signal moved at roughly 1 millimeter per second. The leaves it reached began making jasmonate, a plant hormone involved in defense [3].
If the speed stayed the same, 10 centimeters (about 4 inches) would take about 100 seconds. That is our own arithmetic from the reported speed, not a number from the study. Even so, it shows how a plant can warn its far end within a couple of minutes.
The route surprised the researchers. The signal travels through the phloem, the tubes that carry sugar around the plant, not through any nerve. The trigger is glutamate, a chemical that leaks out of damaged cells and switches on glutamate receptors in the veins. The Saitama release reports that plants missing two of these receptors, called GLR3.3 and GLR3.6, did not pass the signal on [3]. Glutamate is also a messenger in animal brains, which is why the researchers suggested that plants may use their own organs, such as the phloem, together with a signaling system that plants and animals share [3].
Electricity is involved too. The Frontiers review says that after a wound, changes in the electric charge across cell membranes spread to distant tissue together with chemical and hydraulic (water-pressure) signals [4]. So the plant is not using one wire. It is using a team of signals.
What Scientists Still Do Not Know About Plant Signals
Plenty remains open. Thale cress has 20 types of glutamate receptors, and the full job of each is not yet clear [3]. The molecules that decode the flytrap's calcium count were still being searched for [2]. The sources here also do not show that other plants count touches the way a flytrap does, so we should not assume it.
It is also wise not to say that plants "feel pain" or "think." The Saitama team used the word pain only as a possibility for glutamate, and wrote that it "may" act as a damage signal [3]. What is well supported is a plainer idea: plants sense damage and touch, and they pass that information along using electrical and chemical signals.
How You Can Explore Plant Signals Yourself
You do not need special equipment to start. Here are two gentle ways to explore the idea:
- Do the arithmetic. Pick a plant in your yard or classroom, measure the distance from its lowest leaf to its top in centimeters, and work out how long a signal at 1 millimeter per second would need to cover it. Remember this is one speed from one plant, so other plants may differ.
- Watch gently. If a mimosa ("sensitive plant") is available and you have permission, touch one leaf tip once and watch which leaflets fold, and in what order. Please do not poke it again and again, since each move costs the plant energy.
- Read the originals. The Würzburg news pages are short and written for the public, and the Frontiers review is a good next step for older readers.