From The Proceedings of the Athinasius Kircher Society: Linnaeus’s Flower Clock.
Carl Linnaeus, father of taxonomy, divided the flowering plants into three groups: the meteorici, which change their opening and closing times according to the weather conditions; the tropici, which change their opening and closing times according to the length of the day; and the aequinoctales, which have fixed opening and closing times, regardless of weather or season.
Linnaeus noted in his Philosophia Botanica that if one possessed a sufficiently large variety of aequinoctal species, it would be possible to tell time simply by observing the daily opening and closing of flowers. Though Linneaus seems never actually to have planted an horologium florae, or flower clock, his plan was taken up with great passion by many 19th-century gardeners, who often arranged a dozen or more species in the manner of a circular clock face. Below, the approximate opening and closing times of aequinoctal flowers that can be used in an horologium florae: [continue, see image]