L. H. GRAY 



the case in structures such as roots and anthers, and in organs and tissue 

 fragments irradiated in vitro, which derive their oxygen from an ambient 

 fluid. Figure 2 shows the manner in which the mean radio-sensitivity of the 

 cells composing such tissues depends on the oxygen tension at the surface. 

 As would be expected, the larger the structure and the higher the Qq^, the 

 greater the surface oxygen tension required to establish a given level of radio- 

 sensitivity. It had been noted as early as 1934 by Crabtree and Cramer i'' that 

 the sensitivity of tumour fragments irradiated in vitro was somewhat increased 

 when respiration was reduced either by reducing the temperature or by the 



B C 



Lateral Main root 

 root 



D 

 Anther 



E 

 Lymph node 



300 jj 



r=620^ 



r = 350}i r = 550/j. 



C_-. 



200 300 AOO 500 

 Oxygen tension 



700 800 

 mmHg 



Figure 2. Effect of oxygen on sensitivity of tissues 



use of cyanide as a respiratory poison. The fact was confirmed by Hall, 

 Hamilton, and Brues^^, who explained the observation in terms of a falling 

 gradient of oxygen tension from the surface to the centre of the tumour 

 fragment, and showed in support of this explanation that the effect disap- 

 peared if sufficiently small fragments were used. As mentioned above, 

 Kihlman has shown that when respiration is totally suppressed in roots by 

 Cupfcrron, the radio-sensitivity is related to oxygen tension by a curve 

 indistinguishable from that for the cell suspensions so far examined {Curve A, 

 Figure 2, instead of by Curve B, Figure 2). 



Under conditions other than those of total anoxia, temperature may 

 exercise a control over radio-sensitivity indirectly by controlling Qq^., as in 



157 



