892 BIOLOGICAL EFFECTS OF RADIATION 



data obtained by Wyckoff and Luyet, Schreiber, and Oster can all be 

 recorded in the form of typical S-shaped survival curves when the 

 percentage of survivors is plotted against the energy of light used. 

 All these data were recorded for yeasts. Various interpretations may 

 be given to these curves. Some consider the logarithmic portion as 

 denoting a monomolecular reaction. Other environmental factors 

 are considered as responsible for producing deviations from the loga- 

 rithmic type. Others attribute the shape of the curve to multiple- 

 quantum hits on a sensitive region in the cell, while still others consider 

 the curve as merely a normal probability curve due to differences in 

 resistance of individual cells. At the present time the multiple-quantum- 

 hit theory seems to be in favor, although no entity in the cell which 

 might be considered as the sensitive region has been demonstrated. 

 By a mathematical procedure developed by Mme. Curie it is possible 

 to determine the number of quantum hits necessary to kill when the 

 amount of energy and the survival ratio are known. A single hit is 

 defined as the absorption of one quantum in the sensitive region. Hol- 

 weck and Lacassagne (77) determined that nine hits are necessary to kill 

 yeast cells with mixed ultra-violet radiation of wave-lengths of 2800 to 

 3800 A. Schreiber (166) reported that six hits are necessary at 2540 A 

 and 29 at 2290 A. Oster and Arnold (142), however, found that the 

 number of hits necessary to kill is about the same for different wave- 

 lengths in the ultra-violet but that the number of quanta involved 

 in the production of different degrees of inhibition of cell division vary 

 from approximately four to six, and further that the number of quantum 

 hits increases with an increase in the degree of inhibition secured. Hol- 

 weck (76) has gone so far as to calculate that the size of the sensitive 

 zone approximates that of the yeast nucleus. Oster and Arnold (142) 

 expressed the opinion that different quantum hits which they obtained 

 may indicate the existence within the yeast cell of several entities each 

 of which is involved in a future budding. Wyckoff, who has used the 

 multiple hit theory as an explanation of lethal action in fungi, has 

 recently adopted a somewhat modified point of view in his statement 

 that "it is very likely that the quantum picture has a certain reality 

 for electron killing, but it is equally probable that the same order of 

 d-eath from ultra-violet light is an expression of variable susceptibili- 

 ties" (195). 



The Bunsen-Roscoe law holds within narrow intensity limits for 

 yeasts. Oster (140) found that the law is valid for variations in intensity 

 of the incident radiation only up to 30 per cent. Schreiber (166) also 

 found that the law holds within narrow limits, but below these limits 

 a long exposure with a low light intensity is much more effective than 

 a short exposure with a higher intensity. Fulton and Coblentz (54) 

 noted that the Bunsen-Roscoe law also holds for intermittent ultra- 



