Weird and wonderful members of the plant kingdom - Floral Clocks

Linnaeus, the famous botanist and taxonomist (namer of plants), observed that certain plants always opened and closed their flowers at particular times of the day, and that these times varied from species to species. As peasants, who weren't often consulted by scientists, had always know, they could guess the approximate time of day according to which species had opened or closed their flowers. So, arranged in sequence of flowering over the day these flowers could be made to constitute a floral clock or horologium florae, as Linnaeus called it. Linnaeus described three groups of flowers:

1. Meteorici - flowers which change their opening and closing times according to weather conditions.
2. Tropici - flowers which change their times for opening and closing with the length of the day.
3. Aequinoctales - flowers which have fixed times for opening and closing.

It is these latter that he decided could be used to create a clock. However, 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 in real time. However, the idea was taken up by the French composer Jean Français in his composition L'horloge de flore and the following list gives the hour of the day and the botanical and English names of the plants he chose to represent in music:

• 03.00 Day jessamine Cestrum diurnum
• 05.00 cupid's dart Catananche caerulea
• 22.00 Night-blooming Cereus selenicereus grandiflorus
• 24.00 Night-flowering jasmine Nyctanthus arbor-tristis
• 17.00 Moonflower Ipomaea bona-nox
• 19.00 Geranium Pelargonium triste
• 21.00 Night flowering catchfly Silene noctiflora

Various attempts have been made to illustrate the concept, mostly in the form of a clock-face decorated with an image of the flower for the time of day.

Flower Clock photograph by heatkernel, used under a creative commons attribution licence.