In 1729, Jean-Jacques d’Ortus de Mairan (1678-1771) laid the foundations of modern chronobiology. A French interdisciplinary researcher, he designed an experiment demonstrating the existence of a circadian rhythm in plants, potentially deriving from an endogenous clock. These observations and experiments led to the development of chronobiology. I have translated below the original article from 1729 presented by his colleague Mr. Marchant.
De Mairan, J.J.O. 1729. Observation Botanique, Histoire de l’Academie Royale des Sciences, Paris, p. 35.
We know that the “Sensitive” [Mimosa pudica] is a heliotrope [i.e., “sleep in plants”], meaning that the branches and leaves are always directed toward the greatest amount of light, and we know moreover that this property is common to other plants, but it has a more particular one. It is sensitive to the sun or the day, the leaves and their pedicle fold up at sunset, similarly as when we touch, or we shake the plant.
But Mr. de Mairan observed that this phenomenon does not require that the plant be outdoors or exposed to the sun. The phenomenon is slightly less prominent when it happens in a dark environment, but the plant expands significantly during the day and folds up or shrinks regularly in the night. The experiment was done during the summer and was repeated well. The Sensitive feel the sun without seeing it at all. It may have a probable connection with the ability of a lot of sick people to differentiate day and night from their bed. It would be interesting to test whether other plants, whose leaves or flowers expand during the day and shrink during the night, conserve this property in dark environments like the Sensitive.
Figure 1: Experimental Design, light-dark versus constant dark setup.
It would be curious if we could test this property in a hotter or colder oven (incubator), a day or a night that the plant feels, or if we could reverse the day and night and apprehend the order of the phenomena of the real day and the real night, etc. But the ordinary schedule of Mr. de Mairan does not allow him to push these experiments further, and he simply invites botanists and physicists [natural philosophers] to find themselves other things [experiments/hypotheses] to follow [study/research]. The progress of true physics [i.e., science, natural philosophy], which is experimental, can be only very slow.
Figure 2: Original manuscript (in French)
Comments on “Botanical Observation”
This research, performed by de Mairan and presented by Marchant, contains the seeds of modern chronobiology. With this short paper/observation de Mairan and Marchant suggested the existence of an endogenous biological rhythm. He showed that light is not necessary for the oscillations of Mimosa pudica‘s leaves and branches along the day/night cycle; these oscillations persist in the absence of light. However, de Mairan and Marchant were quite careful with their interpretation of the ability of plants to measure time, as they could not rule out other potential confounding factors: temperature cycles, light leaks, or changes in other meteorological parameters. In addition, they already suggested the connection with temperature (compensation/entrainment) using an incubator in a proposed experiment (which was later performed by others), and they suggested a modern experimental design with day/night inversion.
“It may have a probable connection with the ability of sick people to differentiate day and night from their bed.”
Then, in the discussion of the results, de Mairan and Marchant drew an interesting parallel with sick people who can feel the day or night from their bed without actually seeing the light. With this simple comment de Mairan and Marchant insinuated that humans may have an endogenous clock as well.
“The experiment was done during summer and was repeated well.”
The authors were concerned about the repeatability of the phenomena, a prerequisite of both the scientific method and modern statistical thinking. However, the paper does not provide detailed “materials and methods,” so we cannot know if “well repeated” experiments implies that the experiment was done a few times for a day or two, or if the same plants were studied over many days. We have no information on the time resolution (how often the researcher checked the plants). Indeed, de Mairan did not detect that the plants opened up their leaves a little earlier each day––a “free-running” rhythm with a period slightly shorter than 24 hours, which is strong evidence that the rhythm is endogenous.
“He simply invites botanists and physicists [natural philosophers] to find themselves other things to follow.”
As de Mairan’s schedule was packed with many other fields of research (e.g., astronomy), and his responsibilities at the Academie Royale des Sciences, he did not want to invest his time in writing and publishing a paper on a “mere plant.” Thus his experiment was reported by his colleague Marchant. Similarly, de Mairan invites other scientists (“botanists and physicists”) to follow up on these experiments. Many scientists from different backgrounds, such as mathematics, bio-physics, bio-informatics/computational biology, chemistry, medicine, and more, accepted this invitation and enriched the development of chronobiology.
Finally, Marchant and de Mairan concluded the essay by saying that true science is experimental and a very slow process. Indeed, nearly 300 years after his observation, the field of chronobiology is still in progress, and three of its main contributors, namely Jeffrey Hall, Michael Rosbash, and Michael Young, received the Nobel Prize in 2017 for their characterization of the molecular circadian clock. The field of chronobiology will lead to a better understanding of rhythms governing our behavior and physiology.
- Clock Classics: It all started with the plants
- Observation Botanique
- Jean-Jacques d’Ortous de Mairan
- The Basics of Chronobiology
- On the move through time – a historical review of plant clock research
- The Nobel Prize in Medicine Goes to Your Body’s Circadian Clock