Just as humans have an internal body clock that tells us when to go to sleep and to wake up, plants have their own system as well. Carl Linnaeus was able to observe and study this phenomenon when he saw that certain species of plants open and close their flowers at certain times of the day. This led him to develop a sort of floral clock.
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.
(Image credit: Amusing Planet)