uniformity in the cells. The effects of X rays 

 observed at first anaphase were chromosome 

 fusion, fragmentation, and the appearance of 

 achromatic spots. The number of chromosomal 

 abnormalities in buds of various ages reaches 

 its maximum from 1 to 5 days after irradiation. 

 At the time of irradiation the nuclei in these 

 buds were either in the pachytene stage or in the 

 early premeiotic prophase. The chromosomal 

 changes show a direct relationship to the dosages. 

 This linear function signifies that the photo- 

 electrons formed in the absorption of X rays 

 within the tissues act directly on the biological 

 agents. This hypothesis of direct action explains 

 the higher frequency of chromosomal aberrations 

 in cells irradiated at certain stages of division. 

 On the basis of his observations Marshak came 

 to several important theoretical conclusions: 

 pollen mother cells irradiated with small doses 

 of X rays at various stages manifest different 

 frequencies of chromosomal abnormalities at 

 first anaphase. The maximum frequency of 

 abnormalities were observed in buds irradiated 

 when the cells were in pachytene or in preme- 

 iotic mitosis. This indicates that those parts of 

 the chromonemata which are sensitive to X rays 

 are in close proximity in pairs in the pachytene 

 stage and in the early prophase of mitosis. 

 From the nature of the chromosomal configura- 

 tions it is possible to conclude that the chromo- 

 somes consist of two parts when they form 

 configurations, and that the division of chromo- 

 nemata takes place between pachytene and ana- 

 phase in meiosis, and during prophase in mitosis. 



KraevoT (1935) used X rays on young seed- 

 lings of Pisum sativum var. glaucospermum . 

 During the entire growing season samples of 

 the roots were taken from each of the plants at 

 regular intervals. The karyotype of the aber- 

 rants, which resulted from the action of X rays, 

 has unpaired chromosomes and chromosomes 

 whose secondary constrictions are missing. 

 Besides, the chromosomes are somewhat thick- 

 ened. A certain number of aberrants are re- 

 tained until Xj (the second generation of X-radi- 

 ated plants) and for that reason the author calls 

 this type of change delayed modification of the 

 karyotype. KraevoT established a definite regu- 

 larity in the appearance of aberrants. The 

 higher the dosage, the greater the number of 

 aberrants. The number of the latter diminishes 

 with each generation, reaching zero in Xg. 



Heeren, in his work in 1936, once more 

 raises the question of the primary biological 

 action of X rays on living cells since this action 

 usually manifests itself only after a definite, 

 latent, period. Consequently, the determination 

 of what is understood by the latent period is 

 very important. jUngling* proposes to define it 

 as an actual delay in the beginning of the reac- 

 tion. We can delay the beginning of the reaction 

 by means of X rays, if after irradiation of turgid 

 beans we dry them out, thus reducing them to 

 a latent condition, and then soak them again. 



causing them to grow; the action of the rays will 

 be manifest later. This definition of a latent 

 condition was not accepted by everyone. What 

 is usually understood by the latent period is the 

 time that elapses between irradiation and the 

 first visible cellular change. As the methods 

 of investigation are refined, this interval is 

 shortened more and more. In his experiments, 

 Heeren [1936] set out to analyze in greater 

 detail the previously observed retardation in 

 the growth of the plant. Seeds of Vicia faba 

 were subjected to swelling in boiled water for 

 24 hours, after which they were placed in glass 

 dishes filled with water-soaked cotton. A glass 

 plate with grooves was placed alongside the 

 germinating seeds in order to observe the direct 

 growth of the roots by Meissner's method, which 

 is based on the interference of light waves of 

 equal length. This method permitted the deter- 

 mination of lengths of 0. 29m • Before irradiation, 

 as measurements by Meissner's method showed, 

 the roots grew very quickly — 0. 29/1 in 15 sec- 

 onds. When the sprouts were irradiated twice 

 by doses of 9. 5 r the effect of the rays did not 

 become apparent. During the next 2 days the 

 same dose was administered four times and 

 each time immediately after irradiation the 

 growth would cease, only to be followed by a 

 sudden acceleration of growth. These observa- 

 tions show that roots grew much faster imme- 

 diately after irradiation than before. If the 

 dose was increased to 11.5 r, the roots would 

 shorten rather than lengthen. The probable 

 explanation of this phenomenon, according to 

 Heeren, * is that at the time of irradiation the 

 cells always shorten, but this effect is masked 

 by the growth that is proceeding simultaneously. 

 Shortening of the rootlets during irradiation 

 apparently is the result of plasmolysis. 

 Contraction of the protoplast is followed by 

 contraction of the cellular membrane and a 

 shortening of the cells is observed. In order 

 to eliminate the expansion of the root cells 

 due to absorption of water, a paraffin oil was 

 used which does not interfere with the growth 

 of the roots. This experiment permits the 

 differentiation of growth from expansion due 

 to absorption of water and reveals the contrac- 

 tion due to irradiation. Furthermore, Heeren's* 

 experiments laid a foundation for investiga- 

 tion of the effects of various agents on the 

 action of X rays. When a root was narcotized 

 by water saturated with carbon dioxide, it 

 ceased to grow and was incapable of reacting 

 to X rays. These experiments permit us to 

 establish the following picture of the action 

 of X rays: first the protoplast contracts, it 

 withdraws from the elastic cell wall, which 

 follows the contracted protoplast, and then the 

 whole cell shortens. However, cell enlarge- 

 ment by simple absorption of water goes on; 

 X rays have no effect on it. The lack of reac- 

 tion to irradiation in narcotized cells confirms 

 the fact that the active contraction of the 

 protoplast sets in as a result of stimulation 

 caused by the radiation. 



72 



