The plant that counts

The Venus flytrap closes on an insect. That much is familiar. What is less known is that it will not close the first time it is touched.

Inside each half of the trap are small trigger hairs. A single touch produces an electrical signal and nothing else. A second touch within about twenty seconds closes the trap. Fewer than two touches, and nothing happens; more than twenty seconds between them, and the count resets. The plant is, in a functional sense, counting to two and keeping time.

The reason is economic. Closing costs energy, and digesting costs considerably more. A raindrop, a falling leaf or a gust of wind will touch a hair once. An insect walking across the trap touches several hairs in quick succession. Requiring two touches within a short window is a cheap filter that distinguishes food from weather, and it is wrong rarely enough to be worth it.

The counting continues after closing. Three touches make the trap seal tightly. Five trigger the release of digestive enzymes, and the quantity released scales with further stimulation, so a large struggling insect produces more enzyme than a small still one. The plant is matching an expensive response to the size of the opportunity.

None of this involves a nervous system. The mechanism is based on electrical signalling across cell membranes and on the concentration of calcium inside the cells, which rises with each touch and decays over seconds. The decay is the timer. Nothing is deciding anything, and yet the behaviour is precisely what a careful decision would look like.