mutation. In this way the multiplication of 

 genomes prevents the phenotypical appearance 

 of recessive mutations in polyploid forms, and 

 it will be rarer as the number of identical 

 genomes is greater in a plant. In other words, 

 the reason for the lesser mutability of A. bysan - 

 tina, A. sativa, T. dicoccum, T. durum, and 

 T. vulgare is to Ee found in their polyploid 

 stat'e! 



It should be stressed that the polyploid con- 

 dition is significant, in this respect, only for 

 gene mutations. In the case of chromosomal 

 aberrants, Goodspeed (1930*) has demonstrated 

 the reverse to be true: Nicotiana glutinosa and 

 and N. silvestris with 12 pairs of chromosomes 

 are changed by equal doses of irradiation with 

 much more difficulty than N. tabacum and N. 

 rustica, each of which has 24 pairs. For chro- 

 mosomal mutations, the larger number of 

 chromosomes increases the possibility of their 

 occurrence and creates greater possibilities 

 for their variations and survival. On this basis 

 Delone (1930-1931*) obtained many chromosomal 

 mutations by irradiating ears of Triticum 

 vulgare. 



However, in species in which polyploidy does 

 not interfere with the appearance of disturb- 

 ances, variations in the mutability of the genes 

 after irradiation may be observed. Experiments 

 with the irradiation of pollen grains of Antir- 

 rhinum majus in Stubbe's experiments (1929*) 

 showed a frequency of mutations of 4% with a 

 dose of 3000 r. Irradiation of the pollen of a 

 self -pollinating strain A. glutinosum from 

 Opruba with a similar dose did not produce any 

 results. A similar resistance toward X rays 

 is shown by A. siculum. 



If these facts can be substantiated for a large 

 number of species, it will be possible to con- 

 clude that the individual species of a genus are 

 distinguished not only quantitatively, i.e., by 

 the number of their genes or allelomorphs, but 

 also qualitatively, i. e. , by the stability of their 

 allelomorphs. The reason for this must be 

 sought in the process of selection, by means of 

 which labile allelomorphs are gradually cast off 

 by wild forms, whereas the same is accom- 

 plished in cultured species by artificial selec- 

 tion. We must emphasize, however, that up to 

 the present time not a single case is known 

 where the use of high doses of irradiation has 

 failed to increase the number of mutations. We 

 have a right to speak of the genetic action of the 

 rays in general, but we should keep in mind that 

 the quantity of rays needed to obtain a definite 

 percentage of mutations differs for each species 

 of a genus, and, perhaps, for each strain of a 

 species. 



In summarizing the facts about differential 

 sensitivity to X rays the greatest importance 

 must be attached to the ability of various forms 

 to react to irradiation. Different genera, 



species, and subdivisions of species, and even 

 individuals of the same species, react differently 

 to irradiation. Species possessing double or 

 triple the number of chromosomes yield a 

 smaller percentage of mutations than species 

 with a haploid set of chromosomes (Stadler 

 [1929]). Different tissues and cells in the same 

 organism react differently to X rays (the Ber- 

 gonie-Tribandeau Law), as do the various 

 stages of mitosis and meiosis. 



Seeds in various physiological conditions 

 (dry and soaked seeds) exhibit different sensi- 

 tivity to X rays. The greater the intensity of 

 life processes in plants, the greater the effect 

 that irradiation has on them. A definite rela- 

 tionship exists between the water content of 

 cells and sensitivity to X rays. The percentage 

 of mutations is increased when plants are de- 

 prived of phosphorus, and they have a greater 

 sensitivity to X rays in the early stages of 

 development. 



THE BIOLOGICAL DOSIMETER 



As soon as the biological effects of X rays 

 were established, the need for expressing their 

 action in some kind of units immediately arose. 

 Since the action of the rays on human skin soon 

 became apparent (the skin of people who worked 

 with X ray apparatus became reddened and 

 peeled), the so-called erythema dose was pro- 

 posed as a dosimeter. This dose had an inter- 

 national designation HED, an abbreviation of 

 the German words Haut -Erythema -Dosis (the 

 dose which caused the irritation of the skin). 

 However, this reddening varies very greatly de- 

 pending on skin pigmentation (blonds are more 

 sensitive to X rays than brunets)and the age and 

 health of the patient. Consequently, a real need 

 arose for a precise determination of the dosage. 

 This need was stimulated chiefly by physicians 

 who very quickly learned how to use X rays for 

 the treatment of tumors and for diagnostic 

 purposes. We cannot linger on the history of 

 this question. We wish merely to point out that 

 there exist at the present time precise dosime- 

 ters based on the ionization of air by X rays. 

 The number of roentgen units (r) used in experi- 

 ments is determined by means of dosimeters. 

 These dosimeters have determined the extent of 

 inaccuracy of measurement of roentgen units 

 in HED's. Clark's^ experiments, which were 

 performed on numerous patients in several 

 clinics, have shown that, depending on the indi- 

 vidual patient and on his condition, the HED 

 varies between 400 and 1200 r, with the average 

 lying around 849 r. If we recall from the 



^Reference given is incorrect. Can be found in 

 Discussion to Packard, C. 1934. Biological dosi- 

 meters in radiology. Cold ^ring Harbor Symposia 

 Quant. Biol. 2, 264-273. 



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