324 RADIATION BIOLOGY 



factor of 5 or less. These studies are not included in Table 6-1, but some 

 of them are listed among the references and marked by daggers. In 

 Table 6-1, only two items (34 and 73) are recorded with no effect of LET; 

 in both investigations it varied by about a factor of 10. In Figs. 6-1 and 

 6-2 it can be seen that the radiobiological responses of several objects 

 (bacterial spores, fern spores, haploid and diploid yeasts) are practically 

 independent of LET as it varies by a factor of 50 or more, but neverthe- 

 less are markedly influenced within a certain range of this variable. This 

 suggests that practically all radiobiological actions would be found 

 dependent on LET if it were varied over a sufficiently wide range. 



Cases of Decrease in Effectiveness with Increase in Linear Energy Trans- 

 fer. This group includes investigations on an amino acid (item 5), an 

 enzyme (item 7), a number of viruses (items 8-13), vegetative bacteria 

 (items 14-16), a species of yeast (item 21), mutations in fungi (item 29), 

 recessive lethal mutations in Drosophila (items 31 and 32) and chromo- 

 some breaks in Drosophila (item 36). The influence of LET on the 

 mutations and chromosome breaks is not impressive, especially when it 

 is noted that in all cases the low values of effectiveness were obtained 

 with neutrons, for which dosimetry is currently uncertain. However, on 

 the amino acid, the enzyme, the viruses, and the vegetative bacteria, the 

 influence of LET is unequivocal, the effectiveness sometimes varying by 

 factors of 6 (item 16), 8 (items 9 and 10), 20 (item 7), or 30 (item 5). It 

 would appear to be more than coincidental that all these notable decreases 

 in effectiveness with increase in LET are observed in actions on com- 

 paratively simple systems. 



Cases of Increase in Effectiveness with Increase in Linear Energy Trans- 

 fer. Very marked increases in effectiveness are observed for certain 

 actions in such "simple" chemical systems as pure water (items 1-3), 

 water containing molecular oxygen (item 4), and a graphite hydrosol 

 (item 6) ; some of the effectiveness ratios are among the highest listed. 

 Contrasting these with the extremely low ratios mentioned in the pre- 

 ceding paragraph, we note that LET affects chemical actions of radiation 

 as variously (sometimes profoundly) as biological ones. 



The contrast between the responses of vegetative bacteria and bacterial 

 spores (Fig. 6-1) is noteworthy. The biological condition of protoplasm, 

 as might be expected, appears to modify the influence of LET to a 

 marked degree, although it is unfortunate that spores and vegetative 

 cells of the same species have not been simultaneously investigated. 



The response of bacterial spores to LET is almost duplicated by fern 

 spores (Fig. 6-2) but their distinctive curve of effectiveness is not peculiar 

 to dormant cells, since haploid and diploid vegetative cells of Sac- 

 charomyces cerevisiae display similar ones (Fig. 6-1). 



In Drosophila, the production of sex-linked recessive lethal mutations 

 is quite insensitive to LET (items 31, 32); in view of the inaccuracy of 



