Abstract:
Nearly all organisms on Earth are subject, in one way or the other, to the effects of the
planet’s daily rotation about its axis and need to adjust their physiology and behaviour to the
daily cycling of environmental conditions. The observation that living things exhibit rhythms
in their physiology is not a recent one: as far back as the early 18th century, Jacques d’Ortous
de Mairan noted that the common “touch-me-not” plant of the Fabaceae family opened and
closed its leaves once every 24 hours, even when placed in the dark without any light
stimulus (de Mairan, 1729), indicating that something within the plant was producing the
rhythmic phenomenon, rather than it being merely a reaction to rhythmic stimuli. The
clinching evidence for an endogenous source of daily rhythms came nearly two centuries
later however, when Erwin Bünning demonstrated that navy bean plant variants differed in
their endogenous periodicities from 24 h (Bünning, 1935). He correctly deduced that this
meant the plants were not following a rhythmic environmental cue that repeated itself
everyday, as this would produce exactly 24 h rhythms – instead, something from within the
plant was producing a rhythm with a near-24 h period.
