144 



CELLS, TISSUES, AND ORGANISMS 



Figure 7. Tetrahymena grown at 28.5° C. Cells 100 to 110 minutes old were 

 exposed to temperatures between 3° and 37° C. for 30 minutes. The setback 

 (cf. text for Figure 9) changes in a continuous manner with temperature on 

 both sides of the growth temperature, except around 34 to 35° C. In this 

 region the curve for an exposure of one minute to temperatures above the 

 growth temperature is merely shifted in position to show the level part of 

 the cui-ve more clearly. (Data by Thormar, 1956.) 



continues longer and a constant level is reached after 20 to 30 minutes. 

 At the higher temperatures (35°, 36°, 37°, and 38° C.) the setback 

 continues to increase with the time for which the exposure lasts, but 

 the shape of the curves suggests a phase of unsuccessful counter- 

 regulation against this development. It is only at the two lowest tem- 

 peratures mentioned that this counter-regulation, or adaptation, is 

 effective. At constant 31° C. the cells divide after some time, and con- 

 tinued cultural growth is possible at this temperature. At 33° C. the 

 cells divide after a long lag but only once. The divided cell continues 

 to grow for some time. At 34° C. there will never be a division, but the 

 heated cell continues to grow, and after twelve hours ( Thormar, 1961 ) 

 it reaches about double size. At the higher temperatures viability is in- 

 fluenced-at 35° C. after twelve hours, at 36, 37, and 38° C. much 

 sooner. 



Curves which relate age and setback of logarithmic cells are pre- 

 sented in Figure 9. Single cells of varying ages are exposed to tem- 

 perature shocks at 9.3° C. for 30 minutes, at 31.1° C. for 20 minutes, 

 and at 34.0° C. for 15 minutes. Cells less than 30 minutes of age are 



