In Hertwig's experiments (1911) with the 

 action of radium on unfertilized frog eggs, the 

 particular sensitivity of reproductive cells 

 (embryonic tissue, testicles, ovaries, skin) is 

 likewise pointed out. TTie greater the demands 

 placed by life on the cell and especially on its 

 nucleus, the greater are the changes in the 

 nucleus and cytoplasm; life places its greatest 

 demands on embryonic tissues, whose cells 

 grow, multiply, and differentiate into other 

 tissues. 



Schwarz (1913) also raised the question of 

 the stage of development at which irradiation 

 must be given in order to have a beneficial 

 effect. He noted an acceleration of growth only 

 when seeds (before germination) or very young 

 seedlings were irradiated, whereas he was 

 unable to induce this acceleration in mature 

 plants. 



A survey of Kornicke's experiments (1915) 

 indicates that different species of plants display 

 a differential sensitivity to X rays. Moreover, 

 the more intense the life processes of a plant, 

 the more sensitive the plant is to irradiation. 

 In order to ascertain the reasons for the differ- 

 ential sensitivity of different species to X rays, 

 Kornicke formulated the hypothesis that it is 

 related to the number and size of the individual 

 cells, which, in turn, are related to the amount 

 of energy absorbed by them. His experiments 

 indicate that Vicia faba is injured by a dose of 

 100 r, Zea mays by a dose of 250 r, but that 

 even when a dose of 500 r is used, no traces of 

 injury are observed in Papaver somniferum. In 

 addition, the extreme resistance of bacteria to 

 X rays (where the cells, amounts of protoplasm, 

 and water are minute) and the extreme sensi - 

 tivity of large cells such as germ cells and 

 germ -forming cells (which contain large amounts 

 of protoplasm and water, as compared with 

 somatic cells), may likewise indicate the direc- 

 tion in which the reasons for selective radio - 

 sensitivity should be sought. 



Nadson (1922) also points out that radium 

 does not have the same quantitative and quali - 

 tative effect on various species of yeasts, and 

 even on individuals or cells of the same species. 

 "The more intense and rapid are the life proc- 

 esses and development of yeasts, the more 

 sensitive they are to the action of radium; for 

 this reason young cultures are affected by 

 radium more quickly, and to a greater extent. " 



Petry (1921) considers that the experiments 

 of Kornicke and Schwarz have proved that rest- 

 ing (air -dried) seeds are not sensitive to X 

 rays, even if they are subjected to doses 30 [33] 

 times greater than those applied to seedlings of 

 the same species. This proves that the X-ray 

 sensitivity of various parts of the cells is de- 

 termined by the physiological condition of the 

 cell, which must be associated, according to 

 Petry, with chemical differences. However, 



the germination of seeds is not a simple chemi- 

 cal transformation; it consists of many different 

 partial processes that cause changes in the 

 composition and function of the seeds. Conclu- 

 sions concerning the radiation reaction and the 

 insensitivity of resting seeds can be made only 

 on the condition that we know which of these 

 processes is responsible for sensitivity to X 

 rays. The simple swelling of germinating seeds 

 may make them more sensitive to irradiation, 

 just as may the accompanying activation of pro- 

 enzymes. Also, the accompanying enzymatic 

 breakdown of high -molecular reserve substan- 

 ces [assists in the formation of reactive sys- 

 tems] by increasing the number and kinds of 

 molecules as well as by decreasing the size of 

 molecules which increases the ability of sub- 

 stances to ionize. The presence of oxygen as 

 a result of the penetration of water through the 

 seed coat may have an even greater significance. 



Petry also studied the action of other factors 

 in conjunction with irradiation, for example, 

 temperature, cyanide poisoning, absence of 

 molecular oxygen, etc. Experiments showed 

 that X ray sensitivity is not necessarily related 

 to the energy of respiration and growth, and 

 may be independent of them. By its low tempera- 

 ture coefficient Petry considers the X -ray reac - 

 tion not only to be an independent process, 

 independent of the metabolic processes, but, on 

 the basis of this low coefficient, classifies it as 

 a reaction of light. Experiments involving 

 cyanide have demonstrated that X-ray sensi- 

 tivity is not associated with inhibition of growth. 

 From this it is possible to conclude that only 

 the composition of the cells is responsible for 

 their behavior towards the harmful effect of 

 X rays, and not the transformation of substan- 

 ces that takes place within the cell. From these 

 experiments Petry comes to the conclusion that 

 the process of injury to the cells is a purely 

 photochemical reaction. The absence of molec- 

 ular oxygen does not inhibit sensitivity to X 

 rays. The final conclusion from Petry's experi- 

 ments is this: the reason for X ray sensitivity 

 during the time of germination lies not in a 

 change in respiration, but in the commencing 

 changes in chemical composition. The question 

 of whether the swelling of the seeds as such 

 exerts an influence, or whether this swelling is 

 a phenomenon with far-reaching consequences 

 that accompany germination and form a light- 

 sensitive system is still unresolved. The X- 

 ray reactions, therefore, are an independent 

 process, allied to the photochemical one. It 

 consists, in all probability, of purely photo- 

 chemical changes in the important components 

 of the cell. 



Although Perthes' (1923) article was written 

 in order to clarify other problems, it is directly 

 related to differential X-ray sensitivity. His 

 first question is: "do X rays have a selective 

 action?" All the facts point to the differential 

 sensitivity of different types of cells, but 



102 



