ERIK ZEUTHEN AND OTTO SCHERBAUM 



0.5 



■ 10 



hours 

 Figure g. Respiration in populations of Tetrahymena cells plotted on 

 arithmetical scales against time in hours. Open and full circles: three experi- 

 ments with synchronized populations. Upper curve: 10 cells initially present, 

 about i oo cells after the experiment. Second curve from above : 1 1 cells 

 initially present. Third curve from above : g cells initially present, 70 in the 

 end. In the three curves frames indicate division periods observed in the divers. 

 All curves are fitted by eye and in each case two broken lines represent 



± 



Zrf 2 



The fourth curve (triangles) is a control run with 20 cells from 



an untreated population. A control run with empty diver is represented by 

 crosses. (Experiments by Mr. H. Thormar, method of Zjuthen (1953).) 



DISCUSSION 



It is now time to consider what we actually do to the cells, when after growth at 

 optimum or sub-optimum temperature we transfer them for the first time to the cold 

 (7 C.) or to a supra-optimal temperature. In both cases the division index tends to 

 drop after a time which is far shorter than the duration of the cell cycle at the new 

 temperature. This shows that in both cases the rate of entry of cells into division is 

 slower than that of exit from it. Apparently chemical processes occurring in the pre- 

 division stage are exceedingly sensitive to temperature and have a narrower range 

 of optimum temperature, with sharper decline on both sides, than processes under- 

 lying other phases of the cycle. We might also say that they have relatively high tem- 

 perature coefficients below optimum and relatively low above optimum. For this 

 reason the chemical disturbances of cells exposed to cold or to heat might be related 

 to one another, consisting perhaps of changes in the relative concentration of meta- 

 bolites essential for cells to enter a division. Also recovery from exposure to cold or 



J 52 



