146 



CELLS, TISSUES, AND ORGANISMS 



30 60 90 120 



Age since division, nninutes 



Figure 9. Tetrahymena single cells of varying age are exposed to tempera- 

 ture shocks. A: Exposed to 9.3° C. for 30 minutes. B: Exposed to 31.1° C. 

 for 20 minutes. C: Exposed to 34.0° C. for 15 minutes. The growth tempera- 

 ture is 28.5° C. The figure shows the setback in time of division as a function 

 of the age of the cell when exposed to the shock. During a temperature 

 shock a cell may continue to advance toward division, or remain blocked, or 

 may suffer a setback in time. In the first case, the cell is closer to division 

 after than before the shock, and readings will be below the zero line in the 

 figure. In the second case, readings will be on the zero line; and in the 

 third case, the cell is farther away from the next division after than before 

 the shock and the readings are above the zero line. (From Thormar, 1959.) 



slowly through division at the new temperature. In the logarithmic 

 mass population, cells of all ages are present in statistical numbers. 

 When the population is hit by a temperature shock, the older cells lose 

 enough time to be grouped with younger cells, so that after a lag many 

 cells go through division together. And this is so whether the popula- 

 tion is hit by a temperature increase or by a temperature decrease. It 

 would seem that synchronization could possibly be worked out on 

 many temperature schemes. Empirically we have found that the sched- 

 ule already mentioned is the best. 



To Scherbaum (1957a) and to Thormar (1956, 1959, 1961) the 

 temperature setbacks reflect inactivation of a single enzyme. Its re- 

 activation occurs in a standard time at 28° C. and is responsible for 



