 de Candolle also made contributions to the field of chronobiology . Building upon earlier work on plant circadian leaf movements contributed by such scientists as Jean @-@ Jacques d 'Ortous de Mairan and Henri @-@ Louis Duhamel du Monceau , de Candolle observed in 1832 that the plant Mimosa pudica had a free @-@ running period of leaf opening and closing of approximately 22 – 23 hours in constant light , significantly less than the approximate 24 @-@ hour period of the Earth 's light @-@ dark cycles . Since the period was shorter than 24 hours , he hypothesized that a different clock had to be responsible for the rhythm ; the shortened period was not entrained — coordinated — by environmental cues , thus the clock appeared to be endogenous . Despite these findings , a number of scientists continued to search for " factor X " , an unknown exogenous factor associated with the earth 's rotation that was driving circadian oscillations in the absence of a light dark schedule , until the mid @-@ twentieth century . In the mid @-@ 1920s , Erwin Bunning repeated Candolle 's findings and came to similar conclusions , and studies that showed the persistence of circadian rhythm in the South Pole and in a space lab further confirmed the existence of oscillations in the absence of environmental cues . 
