Biology · plants that count
A Venus flytrap's leaf is a pair of red-lined jaws with three stiff hairs on each side. Brush one hair and nothing happens. Brush a second within about half a minute and the jaws slam shut in a tenth of a second. The plant has no brain and no nerves, yet it is keeping count. Try it.
Tap one of the pale hairs inside the jaws.
Each trigger hair works like a tiny lever. When something bends it, cells at its base fire an action potential: a brief electrical pulse, similar in spirit to the signals that run along your own nerves. The pulse sweeps across the whole leaf in a fraction of a second. Yet one pulse on its own does nothing you can see. The trap waits.
If a second pulse arrives soon afterwards, from the same hair or a different one, the trap closes. Researchers put the window at roughly 20 to 30 seconds. Wait longer, and the second touch is treated like a first one. Tap a hair in the simulation, wait for the curve to sag, and tap again: nothing.
How does a plant remember a touch for half a minute? In 2020 a Japanese-German team led by Hiraku Suda and Mitsuyasu Hasebe bred flytraps whose cells glow green when calcium rises (published in Nature Plants). Touch a hair and a wave of glow spreads out from it across the leaf, then slowly dims. Touch again before it has faded and the second wave stacks on top of what is left of the first, climbing higher. Only when calcium passes a threshold does the trap move.
Leave too long between touches and the first wave has drained away, so the second cannot reach the line on its own. That is all the "memory" is: a chemical that takes time to clear.
Plants have no muscles, so how does a leaf close in about 100 milliseconds, one of the fastest movements in the plant kingdom? In 2005 Yoël Forterre, Jan Skotheim, Jacques Dumais and L. Mahadevan filmed traps with high-speed cameras and showed that the secret is shape. Each open lobe is curved outward like a shallow bowl, holding elastic energy. Once the signal arrives, the leaf slowly starts to change its curvature until, past a tipping point, it flips inside out all at once, the way a bent hair clip or a pop-up toy suddenly snaps. Physicists call it snap-buckling. The slow part is the trigger; the fast part is stored energy being released.
Snapping shut is only the beginning. At first the jaws close loosely, the long bristles along the rim crossed like interlaced fingers. A live insect inside keeps struggling and keeps hitting the hairs, and the plant keeps counting.
In 2016 Rainer Hedrich's group at the University of Würzburg (Böhm and colleagues, Current Biology) tracked what each extra signal does. As the count climbs, the touch hormone jasmonate builds up. From about the fifth signal, the trap seals itself tight and glands on its inner surface start producing digestive enzymes, along with the transport proteins that soak up the meal's nutrients, including sodium from the prey's body. The more the prey struggles, the more the digestion machinery is switched on. A twig that sits still never gets past two.
Every snap costs the plant: the trap needs time to reopen, and digestion takes days. Raindrops, wind-blown seeds and falling debris brush the hairs all the time, but usually only once. An insect walking around the leaf touches them again and again. Asking for two touches filters out most false alarms; waiting for five before spending on enzymes makes sure the meal is real and still alive. Try the raindrop button, then let the fly loose and watch the difference.
Charles Darwin, who studied the plant for his 1875 book Insectivorous Plants, suggested a second filter: the gaps between the rim bristles let very small insects crawl out before the trap seals, so the plant doesn't waste a week digesting a snack.
All this cleverness grows wild in only one small corner of the world: the boggy coastal plain of North and South Carolina, around Wilmington.
Sources: Suda et al., "Calcium dynamics during trap closure visualized in transgenic Venus flytrap", Nature Plants (2020) · Böhm et al., "The Venus flytrap Dionaea muscipula counts prey-induced action potentials to induce sodium uptake", Current Biology (2016) · Forterre et al., "How the Venus flytrap snaps", Nature (2005) · Darwin, Insectivorous Plants (1875).