PHYSIOLOGICAL MECHANISM AND BIOLOGICAL 



IMPORTANCE OF THE ENDOGENOUS DIURNAL 



PERIODICITY IN PLANTS AND ANIMALS 



ERWIN RUNNING 



Department of Botany, University of Tiibingen, Germany 



Observation of many physiological phenomena establish that organ- 

 isms are able endogenously to measure the course of the time of day. 

 As far as man and other animals are concerned we call this ability a 

 sense of time, or time memory. Man can estimate the time without a 

 clock, without seeing the sun, in fact, without the use of any external 

 means. Even during sleep, this sense of time continues to function. 

 Many observations establish that lower animals also have such a sense 

 of time. Bees, for instance, can be trained to certain feeding times 

 (Beling, 1929; Kalmus, 1934; Renner, 1957; Wahl, 1932; Fig. 1). 



Plants, also, are able to measure the course of time. With their 

 photoperiodic reactions, the organisms compare the actual lengths of 

 day and night with a measuring stick called the critical day length. 

 The study of these "measures," or in other words, of the light- and 

 dark-induced processes of a specific duration, is one of the principal 

 subjects of photoperiodic research. 



There are reasons for assuming that plants, animals, and man all 

 measure the course of the time of day with the same type of cellular 

 clock system. This physiological clock operates on 24-hr cycles. 



MEASUREMENT OF ENDOGENOUS DIURNAL PERIODICITY 



The physiological clock can regulate several processes and we, 

 therefore, may select different physiological rhythms in order to study 

 the responsible timing mechanism. With constant external conditions, 

 we can, for instance, observe: diurnal fluctuations in turgor-pressure 

 of leaf pulvini, in the growth rate (Ball and Dyke, 1954; Fig. 2), 

 in root pressure, in the activity of several enzymes, in metabolism, in 



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