XIV. X RAYS AND X IRRADIATION 



471 



conditions. These observations have been made on such biologically 

 different materials as plant and animal viruses, bacteria, Dros^ophila 

 sperm, sunflower and mustard seed, mold spores, yeasts, and pnj- 

 tozoa. The graph presented in Figure 7 shows some of these data, 

 together with literature r(>ferences. The logarithm of the per cent 

 survivors is sho\\Ti as the ordinate of the graph. The abscissas repre- 

 sent the X-ray doses in roentgens. It will be noted that the survival 

 curves for each species are quite different, both in their form and in 



100 

 80 



<n 60 



(r. 



o 



> 40 



O 



tr 20 



Id 

 a. 



10 



40000 

 DOSE, 



80000 



Fig 

 rays. 



2000 



DOSE. r. 



7. Survival curves for several species irradiated with known doses of X 

 Forms taken by survival curves illustrate various complexities in X-ray 

 effects requiring Poisson equations of more and more terms to explain them. (1) 

 Drosophi la eggs (43); (2) yeast cells (53); (S) Drosophila sperm (26); (4) colon 

 bacillus (53) ; (5) Drosophila sex-linked lethals (26); (6) ordinary tobacco mosaic 

 virus (23); (7) and (8) Sarrhnromyces ellipsnileus (1.56 and 0.56 A.) (21); (9) 

 Colpidium colpoda (10). 



the amount of radiation necessary to produce a given lowering of the 

 survival curve. As will be seen later, both these factors are important 

 in the interpretation of the radiation effects and the biological changes 

 they make. 



Another type of observation is that of the rate of mutation in the 

 individual genes of an organism when exposed to radiation. Similar 

 data may also be gained for the breakage of chromosomes by X rays, 

 resulting in the formation of inversion, translocation, and deletion 

 in the chromatin structure. Data on lethal mutations in the sex 

 chromosome of Drosophila representing gene mutations and chromo- 

 some deficiencies follow the simplest type of Pois.son equation. Where 



