Light

In 1729 a French astronomer named Jean-Jacques d'Ortous de Mairan noticed something about a plant on his windowsill.

It was a mimosa, one of those plants that opens its leaves during the day and folds them at night. Everybody knew plants did this. The obvious explanation was that the plant was responding to sunlight: light arrives, leaves open; light goes, leaves close. A plant is a simple thing reacting to its surroundings.

De Mairan put the plant in a dark cupboard.

The leaves kept opening and closing. On roughly the same schedule. In continuous darkness, with no light reaching it at all.

I want to sit with how strange that should have been, because it is one of those results that quietly destroys a whole category of assumption.

The plant was not responding to daylight. It was running a schedule of its own, internally, and had been all along. The daylight was not the cause of the rhythm. It was, at most, something the rhythm was set against.

Which means that somewhere inside a plant, with no nervous system and no brain, there is a timekeeping mechanism. Something in there knows roughly what time it is and keeps knowing in the absence of any external information.

That observation sat, more or less, for about two hundred years.

It is worth asking why, and I think the answer is that there was no framework to put it in. What would a clock inside a plant even be made of? The question was not answerable with anything available, and unanswerable questions tend to get filed as curiosities.

It also raised an alternative that took a very long time to fully rule out: that the plant was responding to some other cycle it could still detect in the cupboard. Temperature. Magnetic field. Cosmic rays. Something that varies daily and passes through a cupboard door.

That objection is not stupid, and dealing with it properly is what took two centuries and eventually required putting human beings underground.

Next episode: the bunker.