concerning the mechanism of the action of 

 X rays. 



Muller's first results were quickly confirmed 

 because quite independently Gager and Blakes- 

 lee(I927), working on Jimsonweed [ Datura ], 

 Stadler, working on corn and barley in the same 

 year, and Goodspeed and Olsen (1928), working 

 on tobacco all obtained increases in the fre- 

 quency of gene mutations and chromosome 

 aberrations. 



Even earlier, however, in 1926, Linsbauer's 

 substantial experiments on the action of X rays 

 on mosses and ferns were published. The 

 author selected these plants because, first, 

 their structures are not highly differentiated, 

 and secondly, because of their high regenerative 

 and reproductive properties. He was also in- 

 fluenced by the fact that cell division in them 

 can be observed in vivo . Linsbauer considered 

 that experiments on radiation of these plants 

 could throw light on the physiological basis of 

 the effect of X rays on organisms on the one 

 hand, and could serve for investigation of the 

 mechanism of development on the other. From 

 the mosses he selected: Lunularia cruciata , 

 Fegatella cruciata, Fegatella conica , and Riccia 

 fluitans .~ From the ferns he selected: Lycedium 

 japonicus and Gymnogramme chrysophila . He 

 measured the rays in H. We will leave aside his 

 experiments on the effect of X rays on the 

 sprouting and growth of these mosses and ferns 

 since in the current article we are only inter- 

 ested in their morphological effect. 



Normally in liverworts [i. e. , hepatic 

 mosses], gemmae develop in such a manner that 

 both growing points grow into the long, widened 

 top of the thallus, which bears at its invaginated 

 end an infinitely increasing growing point. When 

 the thallus is several weeks old, it reveals a 

 disklike central part, which is the remainder of 

 the gemma, and issuing from it is a young 

 ribbonlike thallus with dichotomous branching. 



When the gemmae were irradiated by high 

 doses of X rays (from 10 to 40 H), they began 

 to show irregularities in their development, 

 forming thalli with three, four, and more lobes. 

 Sometimes the irradiation can induce true pro- 

 liferation. Linsbauer sees the explanation for 

 such formations in the fact that the irradiation 

 disrupts the development of the embryonic tissue 

 of the growing point, and, although the primary 

 thallus continues to grow for some time, this 

 growth is due only to the expansion of cells that 

 are already formed at the moment of irradia- 

 tion. Eventually the normal growing point 

 ceases to function, thereby providing an impetus 

 for its replacement. Sometimes proliferation 

 is not observed, but the plant remains abnormal, 

 as long as the growing point does not develop 

 and the cells formed around it grow into one or 

 several thalli. Plants closer to the norm which 

 received a dose of 10 H produce gemmae, while 



those which received 20 H have thalli which show 

 deformations. When doses of 40 H are used, 

 substitutions take place after 8 days. It is 

 remarkable that even such a high X-ray dose 

 does not lead either to necrosis of the individual 

 cells nor to dying of the thallus. 



Irradiation of fern prothallia, which had not 

 had a chance to develop very far by the time of 

 their irradiation, makes it possible to observe 

 that some cells do not multiply at all, while the 

 peripheral ones give a considerable number of 

 offspring. Two cells (the eighth and the tenth) 

 in the observed irradiated prothallium produced 

 outgrowths consisting of several cells. From 

 these outgrowths, in place of the original pro- 

 thallium, there develops the replacement for 

 the prothallium, the regenerator [proliferation], 

 which continues to develop further. The ex- 

 treme variant of such changes is a prothallium 

 upon which develop three stalks with embryos of 

 auxiliary prothallia. In addition, it is possible 

 to observe the formation of additional prothallia 

 on the basal cells of prothallia filament. 



Thus, instead of a normal and gradual periph- 

 eral growth, in various places the irradiated 

 thallus produces numerous cell divisions, which 

 give rise to additional prothallia. Eventually, 

 the center of cell formation is displaced, as a 

 result of which separate lobes arise. Further 

 development of the prothallia proceeds normally 

 up to the appearance of completely normal 

 sexual organs. In general, irradiation of fern 

 prothallia leads to the disruption of the harmoni- 

 ous development of the plant, since exposure to 

 irradiation inactivates the cells. 



The experiments of Gager and Blakeslee, 

 published in 1927, were the continuation of 

 experiments reported at the meeting of the 

 American Botanical Society in 1922. Conse- 

 quently, these works should really be considered 

 the first to be set up with higher plants with the 

 specific aim of increasing the number of muta- 

 tions, but a technical point which disqualifies 

 these experiments is the fact that the investi- 

 gators used radium rays rather than X rays. 

 However, the authors used radium enclosed in 

 a tube of lead glass. As is well known, radium 

 emanations consist of a radioactive gas yielding 

 alpha and beta particles and gamma rays. The 

 latter possess the same properties of tissue 

 penetration as X rays, since alpha and beta 

 particles are absorbed by the lead glass; the 

 effect of radium in this case is reduced to the 

 action of gamma rays. 



It should be noted at this point that as far 

 back as 1906 Gager exposed the pollen and ova 

 cells of Oenothera biennis to irradiation. How- 

 ever, the changes produced by these rays had 

 a somatic character. 



From the experiments of Gager and Blakes- 

 lee, of particular interest is one which, for that 



34 



