piastids begin to exhibit an irregular distribu- 

 tion of chlorophyll. Occasionally whole islets of 

 tissue devoid of chlorophyll appear. In older 

 cells the piastids are almost devoid of starch. 

 They are spherical in shape and the chlorophyll 

 is in the shape of thin or thick interconnected 

 bands. Some piastids exhibit an equatorial band. 

 Prothalia with piastids of this type grow very 

 slowly. 



These new plastid types were retained in 

 culture for 6 years. Since changes in piastids 

 were not observed with doses of less than 

 30, 000 r, there can be no doubt that these 

 changes could only have been induced by irradi- 

 ation. In other experiments, performed with 

 millions of prothallia, changes in piastids were 

 not observed. As has been pointed out above, 

 a much higher occurrence of abnormal piastids 

 was observed in Hrub^^'s experiments with 

 Equisetum than in Knudson's experiments. The 

 latter explained the difference by the fact that 

 spores of Polypodium contain less water, while 

 the spores of Equisetum are apparently in a 

 more active physiological state and contain a 

 greater amount of water. As Knudson's detailed 

 experiments have shown, sporophytes obtained 

 from prothalia with B-type piastids (grouped 

 piastids) all had piastids of this type. This 

 grouped arrangement of piastids was maintained 

 for 2 to 3 years, after which this grouping was 

 lost. From young leaves spores were not ob- 

 tained, and from mature ones only gameto- 

 phytes with the normal type of piastids were 

 obtained. Another type of plastid (the giant) is 

 transmitted to the sporophyte and to the subse- 

 quent gametophyte as a result of sexual repro- 

 duction. 



These data almost exhaust the observations 

 of piastids in cells exposed to X rays. This is 

 explained by the fact that the majority of investi- 

 gators directed their attention to changes that 

 arise as a result of irradiation in the nuclei and 

 in the chromosomes. This is quite understand- 

 able since the majority of X-ray biologists were 

 seeking to obtain inheritable changes and conse- 

 quently paid more attention to the nucleus as the 

 carrier of hereditary properties. Investigations 

 devoted to changes in the nucleus and chromo- 

 somes are very numerous, especially in rela- 

 tion to zoological objects, due to the well devel- 

 oped methodology of investigating chromosomes 

 in the salivary glands. However, there have 

 been quite a number of these delicate investiga- 

 tions performed with plants, which often had as 

 their aim the solution of theoretical questions. 

 These will be described below. 



NUCLEAR CHANGES 



Sakamura (1920) was one of the first to 

 investigate the effects of X rays on the nucleus 

 and chromosomes of the plant cell. While 



investigating somatic mitosis in the rootlets of 

 Vicia faba and Pisum sativum , Sakamura came 

 up against certain questions concerning the 

 structure of chromosomes and this caused him 

 to set up broad experiments with the action of 

 various chemical substances and also hot water, 

 electrical fields, and X rays. Of all these 

 effects we are interested here only in the last 

 one. Sakamura demonstrated that when X rays 

 are used, the first effects to be observed are 

 shortening and thickening of the chromosomes. 

 The degree of shortening and thickening depends 

 on the intensity or duration of action. In the 

 same way, chromosome changes are retained 

 longer as the intensity or duration of the X rays 

 is increased. Sakamura also established that 

 the maximum shortening of chromosomes does 

 not take place immediately after stimulation 

 [irradiation], but that a certain amount of time 

 must elapse in order for these changes to 

 appear. If the action of the X rays is too strong, 

 the chromosomes at metaphase will bunch 

 together into a single clump, surrounded by a 

 light area. As soon as irradiation is over, the 

 clumps begin to dissolve and the shortened 

 chromosomes scatter in disorder throughout the 

 entire protoplasm. Since Sakamura never ob- 

 served achromatic filaments during the forma- 

 tion of such clumps, he assumed the hyaline 

 area to be permanently associated with the 

 general disorganization of the achromatic fila- 

 ments. In general, the shape and size of the 

 chromosomes depends on environmental factors. 

 The chromosomes are capable of reacting to 

 different conditions without changing their 

 qualities. 



Iven [1925] after irradiating seeds of Vicia 

 faba, established the frequent appearance of 

 many nucleoli in the nucleus and also of extra - 

 nuclear nucleoli. He also observed deviations 

 from the normal division of nuclei, but not in 

 all the cells. 



Alberti and Politzer (1923, 1924, and 

 [Politzer?] 1934) performed their experiments 

 with animals but their observations and conclu- 

 sions are so interesting that we may include 

 them, as we have done with the experiments of 

 many other zoologists. Alberti and Politzer 

 exposed their materials to 3 -t- 12 HED (first 

 giving them 10-minute and later 40-minute 

 exposures). The objects were fixed after 2, 



4, 7, and 10 hours, and then after 1, 2, 3, 4, 



5, 6, 7, and 8 days. Observing intracellular 

 changes, they established that 2 to 4 hours 

 after irradiation the number of mitoses de- 

 creases and pathological forms appear. Ten 

 hours after irradiation not a single mitotic 

 figure was to be seen. On the fifth day mitotic 

 figures reappeared in material exposed to weak 

 doses. On the basis of their observations, 

 Alberti and Politzer distinguish in the action of 

 X rays: 1) a primary effect, which manifests 

 itself in changes in the course of existing 

 mitoses, 2) an intermediate period without 



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