Biological Clocks Garden Variety Experiments
In plants, the circadian clock controls processes including
leaf and petal movements, the opening and closing of stomatal pores, the
discharge of floral fragrances, and many metabolic activities...
Warwick University, UK biological studies website, 2000.
In 1729, a French astronomer
named Jean Jacques Ortous de Mairan devised a now-classic circadian experiment. Intrigued
by the daily opening and closing of the
leaves of a heliotrope plant, de Mairan decided to test whether this biological
"behavior" was simply a response to the sun. To do so, he confined a plant to
the dark. The daily rhythmic motions of the heliotrope's leaves persisted even
in the absence of sunlight.
Published accounts of de Mairan's experiment led further generations of
researchers to perform circadian experiments with plants. After a long history
of research aimed at understanding the phenomena of circadian rhythms, scientists
are now beginning to identify specific genes, proteins, and biochemical
mechanisms that are responsible for circadian rhythms.
...a rose is not necessarily and unqualifiedly a rose... it
is a very different biochemical system at noon and at midnight.
Colin Pittendrigh, 1965.
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.
Click to watch the video
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
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.
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