Who needs a watch to tell time when we got flowers?

Many species of flowering plants open and close their flowers at specific times throughout the day. The first person to make a recorded observation of this phenomenon was Androsthenes, an admiral of Alexander the Great, who noticed that a tropical Tamarind tree raise their leaves during the day and droop them down during the night. Many people made similar observations—Pliny the Elder, in the first century, and Albertus Magnus, the thirteenth century German bishop.

In 1729, Jean-Jacques d'Ortous de Mairan, a French astronomer, made the first true chronobiologial experiment in history in which he took a Mimosa pudica and kept it under constant darkness inside a closet. In spite of the absence of sunlight, de Mairan was fascinated to observe the daily opening and closing of the leaves. Curiously, de Mairan failed to see—perhaps because he was not a botanist—that this was only possible because of the existence of an internal clock. Instead, the conclusion he drew was that the plant was able “to sense the Sun without ever seeing it”. The concept of internal clocks driving the rhythmic movements in plants developed much later.

It was Carl Linnaeus, the Swedish botanist, who first proposed the idea that it might be possible to tell time with reliable accuracy by observing when certain species of plants open and close their flowers. Linnaeus, who is known as the “father of taxonomy” for developing a system for naming, ranking, and classifying organisms still in use today, had the opportunity to study this phenomenon closely when he was appointed Professor of Medicine at Uppsala University, in Uppsala, Sweden, in 1741, which also gave him responsibility for the University’s botanic garden. Over the next fifteen years, Linnaeus meticulously studied the plants in the garden sometimes working from four in the morning till ten in the evening.

Carl Linnaeus

Ten years after assuming the responsibility of the botanic garden, Linnaeus published Philosophia Botanica, where he compiled a list of a forty-six plants that open during particular times of the day. By arranging these plants in the sequence by which they flower over the day, one could build, a sort of floral clock or horologium florae, as Linnaeus called it.

The following plants were suggested by Linnaeus. The table shows their opening and closing times as recorded by Linnaeus.

Opening and Closing times Plant 3 a.m. Tragopogon pratensis (Goat's-Beard) by 4 a.m. Leontodon hispidum L. (Rough Hawkbit) 4–5 a.m. Helminthotheca echioides (Bristly ox-tongue)

Cichorium intybus L. (Chicory)

Crepis tectorum L. (Hawk's Beard) by 6 a.m. till 10 a.m. Reichardia tingitana (False Sow thistle) 5 a.m. till 12 p.m. Sonchus oleraceus L. (Sow thistle) 5 a.m. till 8–9 a.m. Taraxacum officinale Weber (Dandelion) 5 a.m. till 11 a.m. Crepis alpina L. (Hawk's beard)

Tragopogon hybridus L. (Goat's beard) 5 a.m. till 10 a.m. Rhagadiolus edulis Gaertner 5 a.m. Lapsana chondrilloides L.

Convolvulus tricolor L. (Bindweed, Morning Glory) 6 a.m. till 4–5 p.m. Hypochaeris maculata L. (Spotted cat's ear) 6 a.m. till 5 p.m. Hieracium umbellatum L. (Hawkweed) 6 a.m. till 2 p.m. Hieracium murorum L. (Hawkweed) 6 a.m. till 1–2 p.m. Crepis rubra L. 6 a.m. Sonchus arvensis L. (Field milk-thistle) by 7 a.m. till 2 p.m. Sonchus palustris L. (Marsh sow-thistle) 7 a.m. till 3 p.m. Leontodon autumnale L. (Hawkweed) 7 a.m. till 1–2 p.m. Hieracium sabaudum L. (Hawkweed) 7 a.m. till 12 p.m. Cicerbita alpina (L.) Wallr. (Blue sow-thistle) 7 a.m. till 10 a.m Lactuca sativa L. (Garden Lettuce) 7 a.m. till 3–4 p.m. Calendula pluvialis L. 7 a.m. till 5 p.m. Nymphaea alba L. (White Waterlily) 7 a.m. Anthericum ramosum L. (St. Bernard's Lily) 7–8 a.m. till 2 p.m. Hypochaeris achyrophorus L. 7–8 a.m. till 2 p.m. Hedypnois rhagadioloides (L.) Schmidt

subsp. cretica (L.) Hayck

Trichodiadema babrata (L.) Schwartes 8 a.m. Hieracium pilosella L. (Mouse-ear Hawkweed)

Anagallis arvensis L. (Scarlet pimpernell) 8 a.m. till 1 p.m. Petrorhagia prolifera (L.) Ball & Heywood (Proliferous Pink) 9 a.m. till 1 p.m. Hypochaeris glabra L. (Smooth cat's-ear) 9–10 a.m. till 1 p.m. Malva caroliniana L. 9–10 a.m. till 2–3 p.m Spergularia rubra (L.) J. & C. Presl (Sand spurrey) 9–10 a.m. till 3–4 p.m. Mesembryanthemum crystallinum (Ice-Plant) 10–11 a.m till 3 p.m. Cryophytum nodiflorum (L.) L. Bol. (Ice-Plant) 3 p.m. Calendula officinalis L. (Pot marigold) 3–4 p.m. Hieracium aurantiacum (Hawkweed) Anthericium ramosum L. (syn. Anthericum album) 4 p.m. Alyssum alyssoides L 7 p.m. Papaver nudicaule L. (Iceland poppy) 7–8 p.m. Hemerocallis lilioasphodelus L. (Day-lily)

During the first half of the 19th century, many botanic gardens tried to construct floral clocks based on Linnaeus’s observation but with no great success, because of one important fact Linnaeus was ignorant of—that flowers are highly sensitive to the length of the day, and hence the latitude of the place.

Linnaeus made his observations and measurement in Uppsala, located at about 60 degrees north, where the summer days are long and the nights short. Many of the plants he chose were adapted to those conditions, but would behave differently when taken to another place at another latitude. Furthermore, some of the chosen plants produce flowers regardless of the length of the day, but their times of opening vary with the season, and therefore are not so useful time keepers. Other aspects such as temperature and humidity also influence a flower’s opening and closing times. Under these conditions, it is extremely difficult to make a functioning floral clock.

Incidentally, some 30 years before Linnaeus's birth, such a floral clock may have been described by Andrew Marvell, in his poem "The Garden" (1678):