I originally published this post on May 23, 2007, on the day of the 300th birthday of Karl Linne.

When it's someone's birthday it is nice to give presents, or a flower. Perhaps a whole boquet of roses. But if the birthday is a really big round number, like 300, and the birthday boy is the one who actually gave names to many of those flowers, it gets a little tougher. Perhaps you may try to do something really difficult and build, actually plant, a Flower Clock. After all, it was Carl von Linne, aka Carolus Linnaeus, today's birthday celebrator, who invented the flower clock. He drew it like this, but he never actully built one:

The first one to make (and write down) an observation that some plants (in that case, a tropical Tamarind tree) raise their leaves during the day and let them droop down during the night, was Androsthenes, an officer who accompanied Alexander the Great. In the first century, Pliny the Elder made a similar observation, repeated in the thirteenth century by Albertus Magnus.

In 1729, Jean Jacque d'Ortous de Mairan, an astronomer, not a botanist, reported an experiment - considered to be the first true chronobiologial experiment in history - in which he observed the spontaneous daily rise and nightly fall of leaves of Mimosa pudica kept in a closet in the dark. The experiment was repeated with some improvements by Duhamel de Monceau and by Zinn, both in 1759.

Another Swede, Arrhenius argued that a mysterious cosmic Factor X triggered the movements. Charles Darwin published an entire book on the Movement of Plants, arguing that the plant itself generates the daily rhythms. The most famous botanist of the 19th century, Pfeffer, started out favouring the "external hypothesis", but Darwin's experiments forced him to change his mind later in his career and accept the "internal" source of such rhythmic movements. In the early 20th century, Erwin Bunning was the first to really thoroughly study circadian rhythms in plants. For the rest of the century, animal research took over and though there has been some progress recently, the understanding of clocks in plants still lags behind that of Drosophila and the mouse.

But it was Carolus Linnaeus back in the 18th century who, fond of personifying plants (mostly in regard to sex) named this phenomenon "sleep" in plants. Soon, he switched his focus from movements of leaves to the daily opening and closing of flowers and performed a broad study of the times of day when each flower species opened and closed:

Linnaeus observed over a number of years that certain plants constantly opened and closed their flowers at particular times of the day, these times varying from species to species. Hence one could deduce the approximate time of day according to which species had opened or closed their flowers. Arranged in sequence of flowering over the day they constituted a kind of floral clock or horologium florae, as Linnaeus called it in his Philosophia Botanica (1751, pages 274-276). A detailed and extended account of this in English will be found in F.W.Oliver's translation of Anton Kerner's The Natural History of Plants, 1895, vol.2, pages 215-218. As many of the indicator plants are wildflowers and the opening/closing times depend on latitude, the complexities of planting a floral clock make it an impractical proposition.

While it is not easy to make a functioning flower clock, people have done it. There is one in his hometown of Uppsala, for instance. It has been made in the classroom (pdf) and one can pretty easily find locally useful lists of plants to try to build one.

Linnaeus; in writings titled Philosophia Botanica wrote about 3 types of flowers:1. Meteorici, A category which changes their opening and closing times according to the weather conditions. 2. Tropici, Flowers which change their opening and closing specifically to the length of the day. 3. Aequinoctales, Most important here to this story, are the flowers having fixed times for opening and closing, regardless of weather or season.

It is only those last ones that could be used for buildiing Floral Clocks, while the first two groups were important for the studies of vernalization and photoperiodism in plants in the early 20th century.

You can find some more detail of the flower clock history here. And the idea of a flower clock was also picked up by artists of various kinds: