IM LUENCK OK OXVC.KN ().\ RA1J1()-Sl..\sri 1\ 11\ ( )1 Cl.l.l.S AND IISSUKS 



the experiments l)y Crabtree and Cramer mentioned above, figure 5 shows 

 a very striking example of tiiis. Ciiles" observations'- of the influence ol 

 temperature on the production of chromosome structural damage resulting 

 from the irradiation of Tradescanlia microspores within the anther in an atmos- 

 phere containing 5 per cent oxygen in helium {Figure 2) are contrasted in 

 Fiiinrc J with those of Deschner and myself for suspensions of ascites tumour 

 cells in which /Jq, was held constant at all temperatures. It will be seen that 

 the increasing Qq^ of the microspores themselves, and of other tissues com- 

 posing the anther, steadily reduces the oxygen tension throughout the anther, 

 and hence the average sensitivity of the microspores, until at 40 °C the anaero- 

 bic level of sensitivity is reached. In the case of the tumoiu- cells in suspen- 

 sion, the sensitivity increases slightly with temperature under both aerobic 

 and anaerobic conditions in such a way that the ratio remains constant 

 within the limits of experimental error^. With these facts in mind, we may 



>, 3 



> 



'55 



c 



0) ^ 



in i. 

 > 



JS 

 oc 1 



^ Aerobic 1 anthers 

 A Anaerobic j (Giles) 

 Aerobic 1 cells 

 • Anaerobic J (Deschner and Gray) 



■^•- 



• •-•■« 



.— ^S* 



_L 



10 



20 30 



Temperature 



AO 



Figure 3. Temperature dependence of aerobic and anaerobic sen.siti\ity 



consider the influence of oxygen on the sensitivity of various tissues in the 

 living animal. The relationship for individual tumour cells depicted in 

 Figure 1 and the corresponding constants (m = 3 and K — 5 jaM/l.), refer to 

 irradiation at room temperature. In the living animal we are concerned 

 with the values of these constants at 37 °C. The constant m is practically 

 independent of temperature over the range 3° to 36 °C^, but there are at 

 present no reliable published data for the influence of temperature on K. 

 Our own observations at 36 °C were consistent with a value for /C of 5 fj.M/1., 

 but a value as high as 10 [jiM/l. could probably not have been excluded. 

 Subsequent observations* indicate that A' is the same at 3 ''C as at room 

 temperature and we provisionally assume that at 37 °C the value is the same 

 as at room temperature. If this is so, then it is evident from Figure 1 that 

 the ascites tumour cells would be almost fully sensitive at venous oxygen 

 tension ftaken as 40 mm Hg), and in as far as the sensitivity of other mamma- 

 lian cells is similarly related to oxygen tension it might be thought that all 



* Current investigations by the author and colleagues. 



158 



