Time Relations and Endogenous Rhythms • 47 



light period, and thus darkness during this time promotes flower- 

 ing. However, the effect was not detected in the LDP Plantago and 

 spinach. 



Evidence has also been obtained from leaf movements, a 

 particularly impressive case being that of Madia elegans. This 

 desert composite was first studied by Lewis and Went (1945) who 

 found that it flowered rapidly with 8, 18, or 24 hours of light per 

 day, but slowly with 12 or 14 hours of light. This unusual bimodal 

 sensitivity, with intermediate daylengths less effective than long or 

 short, is apparently reflected in the leaf movements. Bunning (1951) 

 was able to show that these movements corresponded to what his 

 hypothesis would predict for a plant with two photophile phases 

 within each circadian period, and he explained the peculiar photo- 

 periodic response on this basis. Indeed, leaf movements have 

 generally been used as the chief indication of the postulated phase 

 changes. Those in various soybeans, for example, can indicate 

 whether a given variety will show SDP or daylength-indifferent 

 flowering responses (Bunning, 1955). Although leaf movements in 

 Kalanchoe are difficult to detect, Schwemmle (1957) has found that 

 the effects of high temperature given at various times during in- 

 ductive dark periods (see Chapter Two) are well correlated with the 

 effects of similar treatments on the rhythmic movements of the 

 petals of plants in flower. Not all the leaf-movement work is so 

 favorable, however; there is apparently no significant difference 

 between the rhythmic leaf movements of the qualitative SDP 

 Coleus frederici and Coleus frederici x blumei and those of the 

 quantitative LDP Coleus blumei (Kribben, 1955). At best, of course, 

 correlatory evidence is merely circumstantial, whether favorable or 

 unfavorable. 



CIRCADIAN RHYTHMS AND THE ACTION 

 OF LIGHT-BREAKS 



The most widely used tool in assessing the relation of circadian 

 rhythms to photoperiodism, as in the study of low-intensity light 

 processes, has been the light-break. Here, instead of quality and 

 intensity, the timing of the light-breaks and the length of the dark 

 periods are the factors varied. It was tacitly assumed during the 

 preceding sections that light-breaks are most effective when given 



