372 SISTER MARGARET ANN 



It would now seem, however, that while it is true that 

 organisms inherit regulatory apparatus, or " feed-back mech- 

 anisms " which affect the observed rhythmicity, nevertheless 

 these potential mechanisms require first of all to be " set off " 

 by some external environmental factor which is functionally 

 in the normal environment of the animal or plant. This appears 

 in Pfeffer's studies of the so-called " sleep movements " of a 

 certain species of bean seedlings. He found that if the seeds 

 were germinated in the dark, and if the seedlings were kept 

 in the dark, they did not show the " sleep movements." In 

 the natural habitat these " sleep movements " consist in the 

 drooping of the leaves during the night. Pfeffer could, by 

 exposing his " sleepless " plants to a brief period of illumina- 

 tion, cause them to assume the same " sleep movements " as 

 the plants in nature. Even when returned to continuing dark- 

 ness, the plants now persisted in a daily " sleep " rhythm, which 

 consisted of a drooping of the leaves during a part of the 

 twenty-four hour cycle. ^ 



Dr. William Brett has demonstrated instances similar to that 

 of the light-triggered " sleep movements " of the bean seedlings 

 in the case of the emergence of flies from their pupal cases. 

 If kept through their developmental period in total darkness, 

 the flies, whose normal emergence during the twenty-four hour 

 cycle is at daybreak, emerge at any and all hours of the day. 

 But when such dark-adapted larvae were illuminated at a given 

 time with a single flash of light for a period as brief as one 

 minute, the flies then emerged for days after from their pupal 

 cases at exactly that same time in the twenty-four hour cycle. 

 This light-flash was evidently a daybreak-substitute which 

 triggered off the rhythmic emergence of the flies at the twenty- 

 four hour intervals.^" 



As long as purely " endogenous " clocks, located at a nervous 



* W. Pfeffer, Abhandl. sacks. Akad. Wiss. Leipzig., Math.-Phys. Kl., XXX (1907), 

 259, and XXXIV (1915) , 3. Quoted by F. A. Brown, Jr., Sdence, CXXX, No. 3388 

 (1959), 1535. 



^^ Frank A. Brown, Jr., " The Rhythmic Nature of Animals and Plants," Cycles, 

 XI (1960) , 87. 



