DISCUSSION AND CONCLUSTONS There is a large literature, too large for citing here, showing that ionizing radiation produces detectable changes in chromosomes. The following discussion therefore assumes that ionizing radiation causes chromosomal aberrations and attempts to address the possibility that the low-level chronic radiation doses at Site D, Area 11 may have produced an increase in chromosome aberrations in Artemista spineseens there. The occurrence of chromosome aberrancies level radiation exposure of Site D, Area in A. sptnercens 11, NTS, is wel! in the chronic, low substantiated. The fact that the percentages of aberrancies do not meet generally accepted levels of statistical significance for a conclusive difference between the Site D population and our control population raises several questions in interpreting these data, The first question concerns the uncertainty of dose, already mentioned, for those shrubs in Site D. In light of what appears to be a relatively high aberrancy rate in those shrubs where background radiation is "normal," the question of dose must also be considered for those shrubs selected as controls. Since we were not able to obtain, within the time for this work, similar cytological material from regions outside areas of possible radiation doses from nuclear tests in the last 20 years, we have no basis for knowing what the normal frequency of chromosomal aberrancies may be. In addition, there appears to be nothing in the literature concerning the frequency of chromosomal aberrations in plants in nature. Giles (1940) studied species of Tradescantia, a herbaceous genus, {tn the laboratory which "have shown a comparatively high frequency of spontaneous chromosome alterations ..." A more sensitive hybrid used by him had a frequency of 0.3% and the mean for all Tradescantta was 0.11%. Steffensen (1964) also reported on the spontaneous rate of chromosomal aberranctes in Tradescantia. We found a 1.3% rate of occurrence (13 aberrants in 997 cells) and that the radiation dose which doubled this rate was only 0.75 R. (His were acute doses and the effectiveness of acute doses in causing radiation damage at the phenological level and presumably at the chromosomal level is known to be higher than for chronic doses [Sparrow and Puglielii, 1969}]). Nichols, 1941 studied Spontaneous chromosome aberrancies itn onion seedlings where the frequency was variable and high, however "No chromosome aberrations were found in the primary root tip divisions in onion bulbs It interest ."" In short, there appears no Firm basis for evaluating what the spontaneous aberrations rate in Artemtsia might be unless one is willing to compare a woody shrub with herbaceous species in the laboratory. Steffensen's radiation dose for doubling the rate of occurrence of chromosome aberrants, 0.75 8, even though it is an acute dose, is of interest, however, since it suggests that chromosomes are sensitivie to low radiation dose exposures. ts atso of that woody species, generally, are more sensitive to radiation damage than herbaceous species by a factor of about two (Sparrow and Sparrow, 1965). The large variability in the frequency of occurrence of chromosome aberrancies in both the Site D population and the control population also raises some questions as to precisely what the variability in doses may have been to the tissues we have collected for examination, a variability beyond that contributed by the uncertainty of dose exposures to a particular shrub at a particular location with the irregular and uncertain distribution of Pu contaminating the 372 areas around shrubs already mentioned. (It should be pointed out that the possibility radiation exposure from external radioactive contamination of the cells themselves is quite unlikely because of the use of cells from within anthers taken from unopened flower buds. Such cells had never been exposed to the open environment.) If for example a shrub produced branches from its base or from underground, the branches’ exposure to ionizing radiation will have been only for the time it has existed above the surface; and this will determine the dose it will have received. It has, therefore, to be recognized that we are dealing with large variations in radiation doses even from one part of a shrub to another part. From this it follows that a large variability is a likely characteristic of the occurrence of any aspects of radiation effects under conditions of long-term low-level chronic radiation, particularly where the term of the radiation exceeds the life span of the shrub itself, or parts of it. Under these conditions, even if the radiation field were extremely untform, the exposure doses to particular samples taken from vegetation within such a field would undoubtedly be quite variable; hence, variability in any manifestation of radiation effects probably ought to be expected. This kind of variability, of course, does not lend itself to statistical treatment except where very large samples can be taken; and the cytological examination of large samples demands a large expenditure of time and money. It seems probable it is these variabilities that do not allow statistical support for a conclusion that chronic low-level exposures produced increased chromosomal aberrancies. We feel, however, the evidence firmly supports the hypothesis that such has occurred at Site D. The hypothesis that chromosomal aberrancies are indeed caused by ionizing radiation in the field even where morphological changes are not apparent is supported by the occurrence of similar aberrancies in the two other shrubs, Machaeranthera and Krameria from the Rock Valley, NTS experimental ly-irradiated shrub populations. It should be noted that both these shrubs received radiation exposures much higher than the Site D shrubs. Again, however, it can be anticipated that the radiation doses for particular samples taken from the field will be quite variable for different reasons to those for the Site D plants. The Rock Valley samples were irradiated from a central source, a high energy gamma emitter in a stationary position 15 meters above the surface (French, et al., 1974). From this point, exposure doses to shrubs at the ground surface will be varied by shadow effects imposed by other shrubs between the sampled shrubs and the central source, This is particularly the case of lower-growing shrubs like Krameria or Machaeranthera, Although the dose rates in the air above a particular shrub can be predicted and measured with relatively great accuracy and will have a uniformity varied primarily by atmospheric conditions, the ground level doses will no doubt vary widely and wili depend not only on the atmospheric conditions but on the mass of vegetation interposed between the sampled shrub, or parts of it, and the radiation source, The variability in occurrence of aberrancies among our shrub spectes is likely to be different for still other reasons, in part at least, beyond arguments presented so far. The difference in variability between Macharranthera and Krameria, as possibly reflected in the statistical analysis, may be attributable to the phenology of the two shrubs. Machaeranthera, a suffruticose perennial, develops flower heads from tissue which arises, apparently, annually from a low woody base which is itself of varied longevity, and may arise from underground. Kramerta, on the other hand, produces branches, also from underground, but much less frequently on which Flower are borne for several successive years. 373