A Plant in a Dark Closet, Paris 1729: The First Test of Whether Living Things Keep Their Own Day
In 1729 Jean-Jacques d'Ortous de Mairan found that a mimosa kept its daily leaf rhythm in the dark. What he concluded, what he missed, and who finished the argument.
In this article10 sections
In 1729 a plant in a dark Paris closet kept opening its leaves by day and folding them at night, with no sunrise to tell it which was which. The man who watched it, the astronomer Jean-Jacques d’Ortous de Mairan, had stumbled on the first recorded evidence that a living thing can follow a day it cannot see. He did not say the plant had a clock. It took other people more than 150 years to say that.
What did de Mairan find?
Jean-Jacques d’Ortous de Mairan reported in 1729 that a mimosa kept in a dark closet continued to open its leaves by day and close them at night, which made it the first recorded test of whether a daily rhythm depends on a daily cue. The leaves were moving on a roughly 24-hour schedule with no sunrise to prompt them. De Mairan did not conclude that the plant carried a clock. He wrote that it appeared to sense the sun without being exposed to it in any way.
His reading was a reasonable one. In 1729 nobody had a concept of a clock inside an organism. A sealed closet still leaks heat, and the temperature outside rises and falls with the day. Stray light or the day’s temperature swing could each be the cue he had failed to remove, and his wording leaves room for exactly that.
The note itself is short. By the Scientific American retelling it appeared in the Histoire de l’Académie Royale des Sciences under the heading Observation botanique, and de Mairan did not write it up himself. His friend Marchant, who was a member of the Academy, submitted it on his behalf. The account here relies on that retelling and on later histories, since the 1729 text is thin and mostly known through them.
How long did it take to turn one observation into an argument?
It took more than 150 years to get the internal-clock reading into print, and about 240 to anchor it in genes. The sequence below is built from the Scientific American account and from standard histories of the field.
| Year | Who | What changed |
|---|---|---|
| 1729 | de Mairan | Leaf rhythm persists in a dark closet |
| 1758–1759 | Zinn; Duhamel du Monceau | Repeat it while controlling light and temperature; the rhythm persists. Duhamel adds constant high temperature |
| 1832 | Augustus Pyramus de Candolle | Adds humidity control; reports a period near 22 hours in constant light, and flips the rhythm with artificial light |
| 1880 | Charles Darwin | The Power of Movement in Plants argues the plant generates the rhythm itself |
| Early 20th c. | Erwin Bünning | Studies plant rhythms systematically and links them to seasonal timing |
| 1971 | Konopka and Benzer | Fruit-fly mutants with 19-hour, 28-hour and arrhythmic clocks, published in PNAS |
| 2017 | Hall, Rosbash, Young | Nobel Prize for the molecular machinery of the fly clock |
Two rows in that table matter most. The first is de Candolle’s 22 hours. A rhythm that follows the sun should hold 24 hours in any conditions. One that runs slightly off 24 in constant light behaves like an oscillator with its own tempo, and this is the first hint of the “circa” in circadian. The second is Darwin’s 1880 argument, which put the source of the rhythm inside the plant and left later researchers to prove it.
Why does a clock that runs 11 minutes long need an alarm?
The experiment’s logic still governs how any wake-up routine should be judged. A person who wakes at 6:10 every day before the alarm rings may think the body is simply obeying a habit. De Mairan’s closet is the check: remove the cues and see what persists. A change in light through the curtain, the heating coming on, a neighbor’s door, or a truck on the street are all candidate cues, and each of them can make a “natural” wake-up look internal when it is not.
Human beings differ from the mimosa in one measurable way. Czeisler and colleagues at Harvard (Science, 1999) kept volunteers in dim light on a forced schedule and measured the intrinsic period of the human pacemaker at 24 hours 11 minutes on average. That is about eleven minutes longer than the day. Without a daily reset from light, meals, and other people, the clock would drift later each night. A morning alarm is, in effect, the cue that stops the drift. The body’s schedule is already running when the alarm sounds, and what the alarm decides is which hour of that schedule gets treated as morning.
People who sleep two or three hours later on weekends are running a small version of that drift, and the social jet lag research measures the gap between their workday and free-day clocks. The reset cues themselves, light, meals and other people, work best when they arrive at about the same hour each day.
Why the doubt mattered more than the discovery
Popular tellings make de Mairan the founder of chronobiology. A smaller role fits the record better. He designed a clean experiment, reported it with appropriate doubt, and left the interpretation open. It is the doubt that sustained the field for two centuries, because every later researcher had to remove one more possible cue before the claim could stand. Darwin, Pfeffer, Bünning, and Benzer each closed a gap de Mairan had left visible. Twenty-two years after his note, Linnaeus tried to turn the same plant habit into a garden you could read the hour from, and found the weather got in the way.
The doubt also bears on self-experiments today. A tracker that says you “wake naturally at 6:40” has not removed the cues. It has only recorded the time.
Where the closet result stops
The closet leaves several things open. A plant’s leaf movement is not sleep, and the mimosa result says nothing about how long a person should sleep. The 24 hour 11 minute figure comes from a modest number of volunteers under strict laboratory conditions, and a later study from the same group (Duffy et al., PNAS, 2011) put the averages at about 24.1 hours for women and 24.2 for men, so the 11 minutes is one careful estimate and not a constant. The leaf rhythm of a single plant also does not show that every plant, animal and fungus shares one master clock.
Frequently asked questions
Did de Mairan discover the circadian clock?
He ran the first recorded test of whether a daily rhythm needs a daily cue, but he did not claim an internal clock. As retold by Scientific American’s Clock Classics series, his note says the plant seems to sense the sun without being exposed to it. The internal-clock reading came later, with Darwin’s 1880 book The Power of Movement in Plants and the twentieth-century work of Erwin Bünning.
Which plant did de Mairan use?
Most accounts say Mimosa pudica, the sensitive plant, though the 1729 note does not make the species easy to confirm. It suited the experiment because its leaves fold and unfold visibly across a day, so the rhythm could be read without instruments.
What does the word circadian mean?
It joins the Latin circa (about) with dies (day), and it entered science in the 1950s. The “about” matters: human rhythms measured in constant conditions run slightly longer than 24 hours (Czeisler et al., Science, 1999, found an average of 24 hours 11 minutes), so a cue is needed every day to hold the clock to the calendar.
Who won the Nobel Prize for the molecular clock?
Jeffrey Hall, Michael Rosbash and Michael Young shared the 2017 Nobel Prize in Physiology or Medicine for the genes that run the fruit-fly clock. Their work built on Konopka and Benzer’s 1971 paper in PNAS, which found fly mutants with periods of 19 hours, 28 hours and no rhythm at all.
Sources: Scientific American, “Clock Classics: It All Started with the Plants” (A Blog Around the Clock); Czeisler et al., “Stability, precision, and near-24-hour period of the human circadian pacemaker,” Science 284 (1999); Konopka and Benzer, “Clock mutants of Drosophila melanogaster,” PNAS 68 (1971); Nobel Prize committee announcement, 2017.