The flower producing tissue of a branch of Kramerta is therefore more likely to have been exposed more uniformly in time than a branch with a flower head in Machaeranthera, and quite possibly with larger doses for a position equidistant from the radiation source for the following reason. Krameria is more likely to form its own clumps, independent of nearby species compared to Machaeranthera, which often grows under, or within, shrubs with heavier woody stems such as Lyctwn and Ephedra, This particularly contributes to the uncertainty of dose to any particular tissue within Maehaeranthera shrubs. We note in passing that differences in the rate of occurrence of chromosome aberrations at two dose levels for Kramerta is significantly g-eater at higher doses than at lower doses by conventionally accepted statistics. There are still other conditions which support the hypothesis that chromosomal aberrations should be expected even in the low-level radiation areas: 1) High interphase chromosome volumes. Precisely the characteristics which render a plant suitable for cytological analysis, i.e., a smal] number of large chromosomes with a high interphase chromosome volume, characterizes the more radiosensitive species (Sparrow, 1962; Sparrow, et al., 1965). All three species tested here have relatively large chromosomes and should therefore be among the more radiosensitive, Wallace and Romney, 1972 repored that Machaeranthera tortifolia and Atriplex spinescens were the most sensitive, based on phenological or morphological changes, of 31 NTS species studied using gamma radiation in the laboratory. We have already noted that Xrwneria parvifolia in Rock Valley was one of the shrubs responding to increasing exposure rates with reduction in shrub size, more dead stem material, and less foliage on live stems (Vollmer and Bamberg, 1975). 2) Artemisia, particularly, may be more sensitive to radiation effects for yet another reason. The meiotic division was found to be the most radiosensitive stage in the plant's life cycle (Sparrow, 1951); and a pliant which has a long meiotic period would probably be still more sensitive, Such is likely the case for Artemisia which breaks dormancy very early in the spring (it ts among the first to do so); and because of the coo] conditions the meiotic cycle is likely to be longer than for other, later-flowering, species. Thus there appears to be a longer time than for other shrubs during which Artemisia may be radiation sensitive. Unlike carefully controlled laboratory studies of radiation induced chromosome aberrations, it is not possible to know in which stage of the cell cycle or in what part of the plant life cycle the actual radiation "hits” occurred. Sparrow, et al., (1961) noted that in chronic irradiation effects, most observed damage is a result of the previous one or two cell divisions leading to lethal conditions produced by fragments and dicentrics. Aberrations such as heterozygous inversions and translocations, however, are not lethal in somatic tissue because the complete complement of genetic material is maintained, although in These structural rearrangements sometimes produce morphoa different order, logical or physiological changes by altering linkage groups (Swanson, et al., 1967). We noted the occurrence of a high frequency of aberrants indiciating a heterozygous inversion in Machaeranthera (Table 2, Shrub Sample #68), and a a similar high frequency indicating a translocation in Artemisia (Table 1, Shrub Sample #46) If indeed there are radiation induced chromosomal aberrations in Site D, Area 11, according to our hypothesis there is some data which allows some predictions as to long-term ecological effects. Chromosomal aberrations can lead to a general decline in productivity and eventual disappearance of radiosensitive species in an ecosystem (Garrett, 1967; Woodwefl and Sparrow, 1965; Woodwell, 1962; Woodwell and Whittaker, 1968). Wowever, radiation has long been utilized as a method of producing beneficial mutations in crop plants (Smith, 1958; William and Scully, 1961); and it is possible that penetic changes could occur as a result of radiation which might establish individuals or species better adapted to their environment. These adaptations could include radioresistance due to polyploidy, smaller chromosomes, asexual reproduction, and faster mitosis (Sparrow and Evans, 1961; Evans and Sparrow, 1961). With such mutations the resistant species might expand and become dominant. Finally, whether one accepts the hypothesis that there are more than normal chromosome aberrancies in Site D, Area 11, or rejects it depends on interpretation of statistical criteria. The argument has been presented that large varlability should be expected on biological and environmental conditions, and that producing conclusively adequate statistical data requires very large samples which verge on unfeastbility because of the complexity of cytological procedures. 3) Environmental stresses are known to intensify radiation damage, stresses such as heat, cold, and drought (McCormick and Platt, 1962; Woodwell, 1962) to which the NTS species are all subjected. A few words should be said about the detectability of chromosome aberrations. There are no doubt many aberrations that will remain undetected because they occur in cells whose chromosomes are insufficiently spread out for cytological analysis. Some aberrations cannot be detected. These include pericentric inversions, homozygous inversions and translocations, heterozygous paracentric inversions in which crossover did not occur within the inverted loop, and inversions with two strand double cross-over within the inversion loop. Also fragments may be hidden in a group of migrating chromosomes, dicentrics may break early in anaphase, and multivalents are sometimes difficult to distinguish from superimposed bivalents. For all these reasons, the percentages of aberrations detected probably do not present more than an uncertain fraction of the total. Detectable aberrancies are, however, generally acceptable as comparative basis for analyzing populations. 374 375