POLLARD AND KENNEDY 



141 



against the dose. The change from logarithmic to linear increase is significant 

 and probably means, as pointed out by Cohen [8], that some unit is being 

 formed in the irradiated cell which is not suitable for further growth. The 

 cell is then continuing to operate on its existing material and not making fresh. 



With radioactive sulfate the curves shown in figure 4 were obtained. These 

 are the results of a single run, and they are very similar to the uptake curve 

 for phosphate except that the sensitivity to ionizing radiation is not quite so 

 great. In the course of study we noticed that the initial part of the curve did 

 not seem to be as greatly affected by ionizing radiation as later stages, and 

 accordingly a deliberate attempt to study this was made, with the results shown 

 in figure 5. Much higher doses were employed, and it is apparent that the 

 effect of quite high doses on sulfate uptake is to cause a breaking away from 

 an initial line which occurs progressively earlier and earlier in the uptake proc- 

 ess. The final slope of the uptake is diminished as found previously, but the 

 breaking-away point seems to come later and later with lower and lower dose. 



This again seems to indicate that the effect of radiation is on some mecha- 

 nism in the cell that prevents future development of means for further growth. 



2800 



2400 



2000 



1600 



t- 

 z 



D 



o 

 o 



1200- 



800 



400 



CONTROL 



/ 5,300 r 



X/ 43,000 r 



_L 



60 80 100 



TfME OF UPTAKE 



120 WIN. 



Fig. 4. The results of one uptake experiment on sulfate. For low irradiations the ex- 

 ponential character remains, but at higher doses the uptake becomes linear. The uptake is 

 less sensitive than that of P0 4 . 



