A Venus flytrap counts to decide whether the meal is real

Two touches on the trigger hairs, within about twenty seconds, shut the trap. Further touches — about five — start the digestive glands. The tally is a series of electrical impulses.

Each time a hair inside a Dionaea trap is bent, the leaf fires an action potential. One impulse is not enough; a falling drop or a glancing leg does not commit the plant to closing. A second impulse within roughly twenty seconds does, and the trap shuts in a fraction of a second. That much was known. Jennifer Böhm, Rainer Hedrich and colleagues showed that the counting continues after the close. Further impulses, produced while the prey kicks, drive the pumping of sodium and, from about the fifth, the manufacture of digestive enzymes. A quiet trap, shut on nothing that struggles, does not pour out the expensive juice.

‘Count’ here means the plant’s chemistry steps up as the impulses arrive. There is no numeral and no nervous system. There is a threshold, and a second threshold, written in calcium and hormones, that distinguishes weather from food. For a leaf, that is a form of judgment, and it is measurable with an electrode.