By Leon Kreitzman
Gardeners know that plants open and close their flowers at set times during the day. For example, the flowers of catmint open between 6:00 a.m. and 7:00 a.m.; orange hawkweed follows between 7:00 a.m. and 8:00 a.m.; field marigolds open at 9:00 a.m.
In "Philosophia Botanica" (1751), the great taxonomist Carl Linnaeus proposed that it should be possible to plant a floral clock. He noted that two species of daisy, the hawk's-beard and the hawkbit, opened and closed at their respective times within about a half-hour each day. He suggested planting these daisies along with St. John's Wort, marigolds, water-lilies and other species in a circle. The rhythmic opening and closing of the plants would be the effective hands of this clock.
Plants have carefully timed routines determined by internally generated rhythms. In 1729, Jean-Jacques d'Ortous de Mairan, a French astronomer, put a Mimosa plant in a cupboard to see what happened when it was kept in the dark. He peeked in at various times, and although the plant was permanently in the dark its leaves still opened and closed rhythmically – it was as though it had its own representation of day and night. The plant's leaves still drooped during its subjective night and stiffened up during its subjective day. Furthermore, all the leaves moved at the same time. It took another 230 years or so to come up with the term circadian – about a day – to describe these rhythms.
In a similar vein, tobacco plants, stocks and evening primroses release their scent as the sun starts to go down at dusk. These plants attract pollinating moths and night-flying insects. The plants tend to be white or pale. Color vision is difficult under low light, and white best reflects the mainly bluish tinge of evening light.
But plants cannot release their scent in a timely manner simply in response to an environmental cue, like the lowering of the light levels. They need time to produce the oils. To coincide with the appearance of the nocturnal insects, the plant has to anticipate the sunset and produce the scent on a circadian schedule.
Flowers of a given species all produce nectar at about the same time each day, as this increases the chances of cross-pollination. The trick works because pollinators, which in most cases means the honeybee, concentrate foraging on a particular species into a narrow time-window. In effect the honeybee has a daily diary that can include as many as nine appointments — say, 10:00 a.m., lilac; 11:30 a.m., peonies; and so on. The bees' time-keeping is accurate to about 20 minutes.
The bee can do this because, like the plants and just about every living creature, it has a circadian clock that is reset daily to run in time with the solar cycle. The bee can effectively consult this clock and "check" off the given time and associate this with a particular event.
http://judson.blogs.nytimes.com/2009/04/28/guest-column-lets-hear-it-for-the-bees/?th&emc=th
