128 the flowering process 



Manifestations of Biological Time Measurement 



(1,2,8, 12, 15) 



A great many phenomena have been recognized. They seem to 

 fit fairly well into the following four categories: persistent rhythms, 

 photoperiodism, thermoperiodism, and celestial navigation. In a 

 few instances, however, it is quite difficult to classify a given phenome- 

 non. The following outline should give some idea of the breadth of 

 the subject. 



1. Persistent Rhythms 



Probably the most intense interest in biological time measurement 

 is presently centered around these phenomena. Although most of 

 the rhythms approximate 24 hr, only rarely is an exact 24-hr period 

 displayed by an individual organism. More frequently the rhythms 

 deviate up to 1 to 3 hr in either direction from 24 hr. Franz Halberg 

 at the University of Minnesota has suggested the term "circadian" 

 for rhythms which approximate 24 hr in their period. There are 

 many striking examples of circadian rhythms, some of which are 

 listed along with a few comments : 



A. Leaf movements in plants. This may have been the first 

 response noticed, and it has now been studied in considerable 

 detail. Much of the study has been carried out with the common 

 bean plant by Erwin Bunning now at the University of Tiibingen 

 in Germany. 



B. Flower opening and closing, and movement of plant organs 

 other than leaves. Studies of these rhythms have sometimes accom- 

 panied work on leaf movements. Some of these phenomena must 

 have been noticed since time immemorial. 



C. Color changes in certain crustaceans including the fiddler 

 crab. Relatively recent (1940's) studies on the fiddler crab by 

 Frank Brown and his co-workers at Northwestern University in 

 Chicago represent some of the most intensive and detailed work 

 in this field. These color changes will occur for many weeks in a 

 very dimly lit room at constant temperature, according to super- 

 imposed cycles which match the days, the phases of the moon, and 

 the tides at the place where the crabs were collected ! 



D. Respiration and other metabolic processes. Brown now 



