previously cited data the fact that a plant which 

 is sensitive to X rays such as peas is stimulated 

 by a dose of 350 r and depressed by a dose of 

 1000 r, we shall readily understand why such a 

 dosimeter is worthless. 



In this manner accuracy in determination of 

 the dose was attained, but that did not do away 

 with the need for a biological dosimeter. In 

 investigations of the effects of X rays on plants 

 it becomes necessary to grope for the doses 

 capable of inducing desired changes strictly by 

 trial and error. Only by analogy can we judge 

 concerning the effect that the rays will have on 

 a certain plant, and then only approximately. 

 The reason for this lies in the differential sensi- 

 tivity of plants to X rays and in their physio- 

 logical condition. As we pointed out above 

 (Chapter 4, section B), sensitivity to X rays 

 varies greatly with the genus, species, and 

 even variety of the plant. This clearly expres- 

 sed property of plants — reacting variously to 

 irradiation — has forced, and is forcing, inves- 

 tigators to seek a biological dosimeter that will 

 enable them to determine quickly the X -ray 

 sensitivity of a given plant. 



Packard (1931) proposed the use of Drosophila 

 eggs as a biological dosimeter, thereby opening 

 up the possibilities of a new biological approach. 



Johnson (1936) pointed out that two species of 

 plants, Nemophi la and Zinnia, were also sensi- 

 tive to X rays. Although she has not contended 

 that plants could be used as dosimeters, never- 

 theless their extreme sensitivity is of theoret- 

 ical interest because it permits an observer who 

 is experienced in the effects of radiation on 

 plants to judge the approximate dosage on the 

 basis of the extent of injury. The author herself 

 noted that two considerations militated against 

 the use of plants as dosimeters: the variability 

 of the plants and the fact that the effects of irra- 

 diation do not show up immediately. Even if 

 given lethal doses, plants can continue to live for 

 several weeks; leaves, the most sensitive of the 

 organs in respect to rays, can reveal abnormal 

 developments a long time after the irradiation. 



Preliminary experiments indicated that a dose 

 of 2500 r was lethal for 11 -day -old seedlings of 

 Nemophila insignis. In a second experiment, 

 28 -day -old seedlings were irradiated with doses 

 of 1000, 1500, and 2000 r, which showed that a 

 direct relationship exists between dosage and 

 the percentage of survival among the plants. On 

 the one hundred and tenth day after sprouting, 

 70 [70. 8]% of the control plants were alive, 

 60 [59. 7]% of the 1000 r group, 27 [25. 5]% of 

 the 1500 r group, and 6 [5. l]% of the 2000 r 

 group. 6 Thus, the majority of plants which 



^Editor's note: numbers in brackets are the per- 

 centages reported by Johnson (1936). 



received a 2000 r dose died, while those which 

 received 1000 r showed retardation and diminu- 

 tion of growth. A dose of 1500 r exhibited an 

 intermediate effect. The converse is also 

 possible, i. e. , the dose of X rays can be esti- 

 mated by the number and development of the 

 surviving plants. 



Preliminary experiments with Zinnia revealed 

 its sensitivity to X rays. Their effect on the 

 growth, weight, formation of abnormal leaves, 

 and decrease in branches all show a clear rela- 

 tionship between injury and dosage. The size 

 of the dose can be estimated by the extent of 

 the injuries, just as in Nemophila . 



However, since, as the author herself points 

 out, the results require several weeks to show 

 up (110 days for Nemophila ), it is doubtful 

 whether this method can be used as a biological 

 dosimeter. We feel that this question should be 

 approached on the cytological level. Promsy 

 and Drevon (1912), Kornicke (1920), and others 

 mention an increase in the diameter of cells 

 and nuclei due to X -radiation, the irregular 

 course of karyokinesis, etc. This has also 

 been well demonstrated in our work with rye 

 (Breslavets and Afanas'eva [1937]) with peas 

 (Atabekova [1937]), and with wheat (Afanas'eva 

 [1938]). Let us take by way of example the 

 intracellular changes obtained from X -rayed 

 seeds of rye (for a more detailed description 

 and diagrams, see Chapter 3). After irradiating 

 rye seeds with doses of 250, 500, 750, 1000, 2000, 

 4000, and 8000 r, we see the following changes: 

 with a dose of 250 r in 30 roots examined only 

 one nuclear plate was found with a triploid num- 

 ber of chromosomes and one with greatly 

 shortened and thickened chromosomes. In 

 general, the cells and nuclei were completely 

 normal in division as well as in the resting 

 stage. Following a dose of 500 r we observed 

 nuclei that migrated from one cell to another, 

 fusion of cells, and one tetraploid plate in only 

 two roots. A dose of 750 r led to more funda- 

 mental and more numerous changes, including 

 laciniated nucleoli, ring chromosomes, in- 

 creased numbers of nucleoli and chromosomes 

 (two roots were found all of whose nuclear 

 plates had a hexaploid number of chromosomes), 

 syncytia were frequently noted as well as phe- 

 nomena which can only be called cellular bud- 

 ding. With a dose of 1000 r, numerous poly- 

 ploid plates and fragmentation of nuclei into 

 granules were observed. With a dose of 2000 r, 

 more frequent nuclear and chromosomal changes 

 were observed, particularly a shortening of the 

 chromosomes and loss of their satellites. 

 Sometimes, however, the changes were of a 

 more profound nature, e. g. , when the chromo- 

 somes formed a continuous chain or when they 

 became completely shapeless. With a dose of 

 4000 r, very great irregularities show up. The 

 number of fragmented nuclei increases, the 

 nucleoli slip out of their nuclei, and cells of 

 abnormal size appear with large nuclei. A dose 



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