44 • Photoperiodism: Attempts at Analysis 



thus suggested that photoperiodism, in both plants and animals, 

 is merely a special case of a general rhythmic mechanism by which 

 all organisms can register the passage of time. 



ENDOGENOUS CIRCADIAN RHYTHMS IN PLANTS 



Most of the recent data on rhythmic processes in higher plants 

 have come either from Erwin Biinning and his co-workers in Ger- 

 many or from work done elsewhere to test their hypotheses. Biin- 

 ning's concepts (see Biinning, 1956, 1959) have developed from a 

 number of basic observations, some antedating his own work. 



Most plant processes exhibit a diurnal rhythm in phase with 

 the daily alternations of light and darkness. This rhythm is not 

 simply a passive response to external conditions since as expressed 

 in various processes— the nocturnal "sleep" movements of legume 

 leaves, for example— it persists for at least a few days after the 

 plants are placed in a constant-temperature dark room. In fact, 

 periodic light-dark alternations are not necessary to initiate such a 

 rhythm. The classic example is the behavior of bean, Phaseolus, 

 seedlings germinated and grown in constant-temperature darkness. 

 The movements of the young leaves, which can be recorded with a 

 suitable apparatus, are small, more or less random, and unsynchro- 

 nized among the population of seedlings. After a single flash of 

 light the movements become larger, synchronized among all the 

 seedlings, and exhibit a marked periodicity, with the leaves return- 

 ing to the same position about once every 24 hours. The move- 

 ments become weaker after several days and finally die out, but 

 maintain their periodicity until they do. In Biinning's view, such 

 results provide evidence of "endogenous daily rhythms" in plants. 



By "endogenous" Biinning means that the period, or length 

 of a complete oscillation in such rhythms, is determined by the 

 plant and not imposed by external conditions. There are at least 

 three kinds of evidence for this in experiments with the leaf move- 

 ments of bean seedlings. First, of course, the movements are evoked 

 by a single exposure to light, not by a repeated light-dark schedule. 

 Second, the phase of the rhythm— as indicated by the position of a 

 leaf at any given time— is not affected by the solar time of day, but 

 depends only on the time at which the light flash was given. A 

 group of plants given a flash 12 hours before a second group will 



