1264 BIOLOGICAL EFFECTS OF RADIATION 



Before the publication of Muller's results, several investigations 

 of the genetic effects of penetrating radiations in plants were in progress 

 and some positive results had been obtained. The work of Gager and 

 Blakeslee (14) is especially noteworthy in this connection. These 

 investigators treated young buds of Datura Stramonium by fixing within 

 the ovaries tubes containing "radium emanation." Since the alpha and 

 beta radiations were largely or wholly absorbed by the walls of the tubes, 

 the results of the treatment are ascribed to the gamma radiation. A 

 capsule treated at a stage when "reduction had certainly taken place 

 in the pollen mother cells and almost certainly also in the megaspore 

 mother cells" yielded a progeny of 113 plants, among which 20 were 

 found to be chromosomal variants. These included a large number of 

 trisomic forms corresponding to the types previously found in untreated 

 Datura by Blakeslee and his coworkers and shown by them to be charac- 

 terized by the presence of an extra chromosome of the normal comple- 

 ment. The increased frequency of occurrence of these types in the 

 treated series is ascribed to nondisjunction induced by the treatment. 

 In addition there were certain more complex chromosomal variants, 

 notably a new type Nubbin which was shown by Blakeslee (4, 5) to carry 

 two chromosomes of complex structure, each formed of parts of two 

 nonhomologous chromosomes of the normal complement. To account 

 for the occurrence of this type it is necessary to assume that some of 

 the chromosomes in a germ cell of the treated plant had been broken 

 and that the resulting chromosome fragments had become permanently 

 attached in new combination. The proportion of chromosomal variants 

 in the progeny grown from the treated capsule was about 40 times as 

 great as that found in untreated material of the same pure line. 



In the progeny tests of the chromosomal variants it was found that 

 2 of the 18 plants tested were heterozygous for new recessive genes. 

 Apparently in the germ cells of the treated plant gene mutations as well 

 as chromosomal aberrations had occurred. Although the number of 

 mutations found was small, the occurrence of even these few instances 

 was very suggestive, for in the very extensive previous work with the 

 same strain no gene mutations whatever had been found in the progeny 

 of diploid plants. 



Thus the progeny of this single treated capsule of Datura included 

 valiants resulting from three diverse types of germinal variation, namely, 

 change in the distribution of chromosomes, internal reorganization of 

 chromosomes, and change in individual genes. Subsequent investigations 

 have shown that these three types of germinal variation are induced 

 by high-frequency radiation in a wide variety of forms, both plant 

 and animal. 



The study of the genetic effects of irradiation in Datura has been 

 continued and extended by Blakeslee and his associates. This work 



