TIMING AND THE FLOWERING PROCESS 151 



have been tested for effects on the period of certain rhythms, the 

 relatively very few which were effective, such as cobakous ion, ether, 

 various narcotics, etc., may or may not influence the nucleus. 



A critical experiment with the green algae Acetabularia has recently 

 been performed. This is a single celled organism large enough that 

 the nucleus can be removed, but the functions of the cytoplasm will 

 continue for a while. In such an experiment, time measurement (a 

 rhythm in photosynthesis rate) continued in the cytoplasm in the 

 absence of the nucleus. Thus in Acetabularia, at least, timing is not 

 directly dependent upon the nucleus. Can we extend this finding to 

 all organisms ? 



4. How Does the Clock Work? 



Many physical models have been suggested to account for and to 

 help explain some of the observational data. Bunning explains some 

 observations by assuming that the biological clock resembles a 

 relaxation oscillator. Low temperature and other treatments may 

 be effective during only a part of the cycle and not during the other 

 part. Bunning suggests that the effect can be observed only when the 

 oscillator is becoming tense. 



Colin S. Pittendrigh, and V. G. Bruce at Princeton (1, 18) also 

 propose a rather elaborate physical model designed to account for 

 the experimental observations. Their model, although it is quite 

 complex, is an extremely elegant and impressive one. It accounts for 

 a number of rather detailed observations including some which have 

 not been discussed here. For example, when a rhythm is readjusted 

 by some environmental stimulus such as a light flash or sharp 

 temperature change, the new rhythm may be assumed after several 

 short or several long cycles. Such short or long cycles are called 

 transits. 



Yet the model is typically a physical one, and we are still left with 

 no biochemical understanding of the timing process. Discovery of 

 these mechanisms could very well constitute some of the most 

 exciting and interesting research in the future of biology. Probably 

 whole new concepts will arise as we increase our understanding. It 

 is not unreasonable to imagine that the principles learned about 

 timing in living organisms will be comparable in interest and funda- 

 mental importance to the recently acquired knowledge about the 

 mechanism of action of the genes. 



