SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 



145 



blocked by 9.3° C, continue to advance at 31.1° C, and are set back 

 when exposed to 34° C. All cells older than 60 minutes are set back 

 by all three kinds of shocks. For each temperature there is a smooth 

 transition from young cells with low reaction to old cells with strong 

 reaction to the temperature shocks. Just before the macronuclear 

 stretching and cytoplasmic division, there is an abrupt change in 

 response to the increased temperature. The cells are either lightly set 

 back, blocked, or (as shown by the points below the zero line) go 



20 



30 40 50 60 70 



duration of heat shock, min. 



Figure 8. Tetrahymena pyriformis grown at 28.5° C. and shocked at the 

 age of 100 to 105 minutes. The graph shows the relation of the excess delay 

 (ordinate) to the duration of the temperature shock (abscissa). Upon ex- 

 posure to 34.1° C, the excess delay reaches a maximum of about 100 min- 

 utes after 20 minutes. At lower shock temperatures the excess delay reaches 

 an earlier and lower maximum and shortens upon longer exposure. This is 

 evidence of adaptation to the shock temperature. At higher shock tempera- 

 tures the curves suggest at least two effects— separable on the time scale— of 

 the elevated temperature. (From Thormar, 1959). 



