De Mairan's Experiments. De Mairan hesitated to conclude that heliotropes have internal clocks. He could not rule out other possible factors, such as changes in temperature or magnetic forces. De Mairan's findings might have gone unnoticed, had a colleague not published the results for him.

A Clock Made of Plants. Carolus Linnaeus, an 18th century Swedish naturalist, developed the genus and species naming system for organisms. He also devised a living clock — often illustrated though perhaps never actually planted — which used the opening and closing of flowers on living plants to mark the hours of the day.

As Different as Day and Night. This video shows circadian leaf movements similar to those noted by de Mairan. The Albizzia julibrissin plant shown here is one of many plants that, under constant conditions, folds up its leaves at night and opens them again in the daytime. A motor organ called the pulvinus is responsible for the movement.

Darwin — Taking the Measure of Plants. Charles Darwin, best known for his theory of evolution, was among those who pursued de Mairan's findings. Darwin designed an apparatus for measuring leaf movements, and quantified, graphed, and published his results in The Power of Movement in Plants, a book he wrote with his son.

Darwin's Drawings of "Sleeping" Leaves. These illustrations of various plant species are from Darwin's The Power of Movement in Plants. Darwin described the nighttime, folded-leaf state of plants as "sleep," and hypothesized that it was a way for plants to reduce exposure and thus conserve energy. Current research suggests that, among other possible advantages, the nighttime position of leaves may indeed be a way for plants to conserve heat and/or water.

As Time Goes By — Circadian Research Circa 1999.
This diagram, published by Jay C. Dunlap, shows the range of organisms in which circadian systems have been investigated. The circadian rhythms of the groups in blue have been studied at a physiological level. The groups in red represent those in which the clock mechanism has been studied at a genetic and molecular level. The scaled line lengths correspond to evolutionary distance between phylogenetic groups.

A Model Genetic Plant — Arabidopsis thaliana. This small, short-lived species of mustard plant exhibits visible circadian rhythms. It has proved to be a particularly useful organism for circadian research, in part because its approximately 120 million base-pair genome has been nearly completely sequenced.

Arabidopsis Genomics.
These cover illustrations show mutant and normal Arabidopsis thaliana. To go beyond mere appearance, researchers insert the firefly luciferase gene, responsible for that insect's characteristic glow, into Arabidopsis. The luciferase gene serves as a "reporter" in that it produces a protein that results in bioluminescence if the neighboring gene or genes are expressed. Using that method, researchers have recorded patterns of gene expression (see illustration at position nine o'clock). The patterns of certain genes follow a circadian rhythm.