1316 BIOLOGICAL EFFECTS OF RADIATION 



at least two absorptions before death takes place. The published 

 survival curves for organisms like staphylococci which experience has 

 shown to be difficult, if not impossible, to separate into individual cells 

 before irradiating seem to have a like interpretation. The many-celled 

 nature of the material also affects the curves for small pieces of tumor 

 tissue, bean, sunflower, and mustard-root tips (21, 21a), or Drosophila 

 eggs. 



Other multiple-absorption curves apparently cannot be accounted for 

 by the numbers of cells which compose the objects irradiated, since the 

 objects seemed to be in a single-celled stage [yeast (20), alga (54a), 

 Rhizopus spores (55), ascaris eggs (6), and the protozoan Colpidium 

 colpoda (10)]. In accounting for the survival curve of Colpidium, 

 Crowther (10) showed that the long delay in the time between the com- 

 mencement of exposure to the X-rays and the beginning of the deaths 

 was explicable on the assumption that more than one absorption in the 

 vital spot was necessary to cause death, the least number of absorptions 

 which he considers sufficient being 49. In yeast, Glocker, Langendorff, 

 and Reuss (20) consider five absorptions necessary to kill. For alga, 

 Langendorff and Reuss allow three absorptions, and for Ascaris four to 

 six are considered necessary in the Braun and Holthusen data (6). 

 Illustrations of these multiple-absorption types of survival curves are 

 shown in Fig. 2. 



Glocker and Reuss (21) have utilized both hypotheses, the many- 

 celledness, and the necessity for multiple absorption, to explain the 

 mechanism by which certain of their experimental survival curves are 

 derived. They have also adopted the idea that the wave-length of the 

 incident beam plays a part in the severity of the effect produced. In 

 data on the root tips of the horse bean, sunflower, and mustard seeds, 

 they were able to show a difference between survival curves of these 

 objects when the wave-lengths of the incident beam were 0.56 and 

 1.54 A. These curves were centered on the point of 50 per cent survival 

 and the dose scale converted to a ratio of the actual dose divided by the 

 50 per cent, q/}/2Q- At the shorter wave-lengths the roots died more 

 slowly at first, then more rapidly than those of the 1.54 A. The differ- 

 ences are generally small, though rather consistent. The method of 

 presenting the data only in graphs does not allow the estimation of their 

 statistical significance. The effect of the wave-length is not seen in 

 other forms, bacteria, yeast, an alga, .4 scam, or Drosophila eggs. 



The wide variation in the survival curves of the different species 

 (Fig. 2) shows that pronounced differences exist between living forms and 

 their reactions to irradiation. This variation in the sensitivity of species, 

 due to the normal biological variations of inheritance or environment, has 

 suggested to several investigators that like, though possibly smaller, 

 biological variations account for the form of the survival curve within 



