Day and night effects on the animal and plant kingdoms: The eve of chronobiology

Abstract

ABSTRACT For a long time, cyclical changes in the body were assumed to be caused by the cyclicity of the environment (day-night, seasons). The concept of daily and seasonal changes was first documented in the 18th century by astronomer D’Ortous de Mairan, who demonstrated that plant leaf motions varied depending on the time of day, and by Linné’s description of his floral clock in 1751. In 1832, De Candolle was the first to experimentally establish the endogeneity of rhythms in plants, underlining the notion of what we now term free-running rhythms. Julien-Joseph Virey made his own contribution in his thesis, published in 1814, against this backdrop, in which he examined the knowledge of his day on the daily and seasonal biological fluctuations of living matters. He emphasized the relevance of the environment’s day-night cycle on plant life and created a list of plants based on their diurnality or nocturnality. He expanded on the issue of rhythmic changes in human health and sickness and provided his own data on the daily fluctuations in patient mortality he discovered at the Val-de-Grâce military hospital where he was chief pharmacist. What is crucial is his use of terms such as “living clock,” “entrainment,” and “innate rhythm” and the applicability of the advanced concepts Because Virey introduced the notion of temporal variations and the impact of the alternation of day and night on these variations, this thesis is a historic testimonial to understanding of biological rhythms in the first half of the 19th century. We may assume from his writings on rhythmic fluctuations that he offered the theory, followed by an experiment, however primitive, from which he drew conclusions and postulated a mechanism (the living clock) that would later prove accurate. All of these aspects indicate that this study represents an early exploration of the notion of temporal variations in humans.