Relationships between photoperiodism and circadian rhythms of activity in the house finch.

Under field conditions, a vast majority of the behavioural and physiological properties of an organism show rhythmic variation with a daily repetition. Although some of these rhythms may be passive systems, driven by the external environment, an increasing body of evidence indicates that, instead, most of the daily rhythms appear to be endogenous, and are merely synchronized by the environment. Under constant laboratory conditions, the periods of the rhythms usually deviate consistently from the exact 24 hr. value seen in the field. Such rhythms are now commonly designated as 'circadian' (Halberg, Halberg, Barnum & Bittner, 1959), and an extensive literature has developed describing their general properties. The endogenous origin of daily rhythms has been appreciated for many years; the experimental analysis of rhythms in plant-leaf movements has a particularly long history (Biinning, i960). Many organisms, in addition to daily rhythmic patterns, also exhibit seasonal rhythms. In 1920 Garner & Allard discovered that daylength was involved in the control of seasonal floral morphogenesis, a discovery which stimulated extensive experimental work in photoperiodism and eventually led to the conclusion that daylength was directly responsible for the timing of the seasonal responses of many plants, insects and birds. Impressed by the fact that light cycles can synchronize plant leaf movements, and that daylength controls flowering, Biinning proposed in 1936 the hypothesis that 'the physiological basis of.. .photoperiodism lies with the endogenous daily rhythms...'. Further, he suggested that an essential step in the study of plant photoperiodism was an analysis of leaf movement rhythms, because these are an easily measured expression of the many other internal changes that are equally 'regulated by the endogenous daily rhythm'. Implicit in this hypothesis are two major tenets: (1) that a daily endogenous rhythm times the photoperiodic response; and (2) that there is one 'master-clock', and that an observer can therefore 'read the hands' of that clock by examining any endogenous daily rhythm of the organism in question.