The Life Cycle of the Single Cell 21 



It was found that the rhythms could be altered in three ways. First, 

 the frequency, normally 24 hours, could be changed over a wide range. 

 For example, the organisms were exposed to alternating periods of 6 hours 

 of light and 6 of dark and their rhythms speeded up until they repeated 

 every 12 hours instead of 24. Other periods were attained by appropriate 

 changes in the duration of light and dark periods. However, no matter 

 how long the different regimen was imposed, as soon as they were re- 

 turned to constant dim light, the cells resumed their 24-hour cycle ( lower 

 curve of Fig. 9). Second, the rhythm could be wiped out entirely by in- 

 cubating the cells in constant bright light. In this condition, the cells 

 photosynthesized all the time, reproduced asynchronously at any time, 

 and luminesced at no time. Again if placed back in constant dim hght or 

 total darkness, they promptly resumed the normal rhythmic pattern. 



Third, the rhythm could be reset. That is, the "clock" could be 

 pushed ahead or set back. This happened when cells were incubated in 

 constant darkness and suddenly were exposed to strong light for a minute 

 or two. Before exposure the cells had marked off regular 24-hour cycles. 

 After exposure they continued to do so but in a different time sequence, as 

 if one suddenly moved the hands of a watch forward or backward. The 

 light signal acted as the "reset" mechanism. 



We have stressed these rhythmic activities because many living 

 organisms are rhythmic, and the rhythms profoundly affect their develop- 

 ment, indeed, are a part of it. Not all organisms operate a rhythmic cycle 

 of 24 hours' duration. Some have a tidal rhythm, some a lunar ( 28-day ) 

 rhythm. Others have an annual rhythm, and a few have several different 

 rhythms at the same time. Consider the following examples. 



The fiddler crab, a common seashore denizen, changes color mark- 

 edly. The change is rhythmic and keyed to the tide. The same is true of 

 oysters and clams, who open and shut their shells according to a tidal 

 rhythm. Some insects pupate according to a 24-hour rhythm. That is, they 

 emerge from the pupal case at a certain time of day and only at that time. 

 The mating habits of certain fish are rhythmic according to a lunar cycle. 

 The hamster and rat show precise daily cycles of activity when placed 

 in special running cages (with automatic tachometers) for long periods 

 of time. The growth rates of some microorganisms show a similar daily 

 rhythm. Finally, the menstrual cycle of the human female has a lunar 

 rhythm. All of these, save the last, have been studied in the laboratory and 

 show the same characteristics as the rhythms of Goni/aulax. Let us sum- 

 marize these characteristics: 



1. The rhythm is keyed to the physical environment of the organism 

 ( day, tide, lunar month, year, etc. ) and probably enables the organism to 

 feed or mate or protect itself more efficiently. 



