of 8000 r causes still more pronounced irregu- 

 larities. The nuclei become completely disrup- 

 ted or else they grow buds or assume abnormal 

 shapes. Hence, each of the doses of X rays 

 used by us induced definite intracellular changes, 

 and, conversely, the number and kind of changes 

 made it possible to estimate the size of the dose 

 used. This suggests that intracellular changes 

 make the best biological dosimeter. They corn- 

 pletely satisfy the criteria set up by Rubinshtein 

 (1933) for biological dosimeters: 1) availability 

 of the object in numbers, 2) uniformity of the 

 material, 3) sensitivity to reaction, 4) accuracy 

 of reaction, and 5) independence from external 

 factors. To these criteria another one may be 

 added, which was proposed by Johnson, namely, 

 the speed of reaction. On the third or fourth 

 day after the irradiation of seeds or on the sec - 

 ond day after the irradiation of seedlings we can 

 already have our answer on how a certain plant 

 reacts to a given dose of X rays by its intra- 

 cellular changes. To determine these changes 

 by developing a method for crushing the roots 

 and staining them with acetocarmine does not 

 present any difficulties. 



RETENTION OF THE EFFECTS OF X RAYS 



In the great majority of experiments with 

 irradiation of seeds, the latter are planted im- 

 mediately after irradiation. Naturally, the 

 question of how long the effect can be retained 

 comes up. Very frequently for theoretical pur- 

 poses and particularly for practical ones it is 

 important to retain the effect for a long time. 

 Our laboratory has often received requests to 

 irradiate seeds from outlying places where good 

 X ray facilities and dosimeters do not exist. If 

 the effect of X rays were retained for only a 

 short time, it would obviously be impossible for 

 us to accept such requests. Since the literature 

 on this question was very contradictory, it be- 

 came necessary to set up our own experiments 

 to clarify it. 



Guilleminot (1910) writes: "The injurious 

 effect of irradiation is retained for two years in 

 its original force. " 



Schwarz (1913) considers the effects of X rays 

 to be reversible, i.e., irradiated seeds retain 

 the effect induced by X rays for only a short 

 time. 



This opinion is seconded by Geller [1924] who 

 concludes, on the basis of his experiments, that 

 the irritating effect of X rays is transitory and 

 replaced in a few days by normal development 

 unless the irritation is such that it leads to 

 gradual death of the plant. 



Iven (1925) tries to prove, on the basis of 

 experiments of Guilleminot [ 1910] and Weber 

 [1923], that even after several months of 



storage irradiated seeds retain the full force 

 of the primary effects of irradiation. 



The experimental data of Muller'^ (1928) and 

 Harris (1929*) on this subject are very valuable. 

 They pointed out that the frequency of lethal 

 mutations was identical in sperm taken immedi- 

 ately after irradiation and in those taken several 

 days afterwards. 



Afanas'eva (1936 and 1938)* used a plant with 

 which she had done a great deal of work, namely, 

 spring wheat Triticum vulgare var. Caesium 

 0111 (pure strain), the seeds of which were 

 irradiated with a dose of 16, 000 r. The author 

 was aware from previous experiments that this 

 dose invariably produced the same changes in 

 the cells: inclusions in the cytoplasm, forma- 

 tion of micronuclei, occurrence of two or more 

 protoplasts within single cells, changes in the 

 number and shape of the chromosomes, irregu- 

 lar nuclear plates, and, finally, chromatin 

 bridges. In the first experiment the seeds were 

 planted at five different periods: one day after 

 irradiation, I month later, 3 months later, 6 

 months later, and a year later. Table 39 indi- 

 cates that the various changes described above 

 appeared after a day and also after all the other 

 periods including 1 year. 



No reduction or disappearance of the effects 

 of X rays was observed. On the contrary, there 

 was a certain increase in the various changes, 

 e. g. , the number of micronuclei and multi- 

 nuclear cells increases somewhat. Three 

 months after irradiation the number of micro - 

 nuclei was 25 per root instead of 12, 6 months 

 later it was 31, a year later the number went 

 down again close to the original figure. 



It seemed interesting and worthwhile to 

 determine whether the effect of irradiation could 

 be retained for even longer periods, say for 2 

 or 3 years. Simultaneously with the planting of 

 the seeds which were irradiated in 1934, two 

 controls were planted. The first control con- 

 sisted of unirradiated wheat seeds of the 1933 

 crop, which were kept under the same conditions 

 as the irradiated ones. Seeds of the same pure 

 strain of the 1935 crop served as the second 

 control. In the second table (Table 40) data of 

 the cytological examination of 1937 are com- 

 pared with those of 1934. 



Table 40 shows that the effect of X rays on 

 the cells of seeds stored for a long time were 

 not lost. All the intracellular changes induced 

 by X rays and observed in roots fixed several 

 days after irradiation were also observed in 

 roots fixed 3 years after irradiation. The num- 

 ber of observed changes, the number of micro - 

 nuclei, in particular, actually increased, as did 



^Editor's note: See Muller reference on page 321, 

 we have been unable to confirm this reference. 



109 



