ar wn ve oe oy : netaer —* Fig. 5. chants Be RA hae ee nnn Sw RLS GRERMS EL mE * eS The arrow points to a ring chromatid in Artemisia spinescens. Fig. 6. The species, Machaeranthera tortifolia came from the experimentally irradiated plot in Rock Valley, NTS. This species is characterized by six chromosome pairs consisting of three large and three slightly smaller pairs (Fig. 6). A large number of extranuclear bodies occur in telophase I and telophase If (Fig. 7), although they are not as large or prominent as those of Artemisia spinescens. Flower buds collected 14 and 29 April 1976 contained meiotic cells at the proper stage for analysis. Chromosomal aberrations detected included five bivalents and two univalents, acentric fragments, and dicentrics (Figs. 8, 9, 10, and 11; Table 2). No multivalents were observed. Of the The normal condition of six large bivalents in Machaeranthera torttfolia. came from an area remote to the radiation source where the radiation level It has been included with the lower level for analysis, howwas background. A large part of the aberrant cells from the high dose area came from ever. a single plant which contained 64% of the anaphase cells showing a dicentric and/or a fragment (Fig. 11) showing that the plant was heterozygous for a paracentric inversion. 2,714 cells analyzed, 1,767 came from nine plants with doses ranging from 2.5 The Mann-Whitney test does not indicate significant differences between these populations. ated area where the dose rate was less than 1.0 R/day. The third species krameria parvifolia which came also from the Rock Valley to 4,5 R/day, and 947 cells were from five plants outside the immediate irradi- One of this latter group site, 360 has six large chromosomes (Fig. were collected on 19 and 20 May 1976. 361 12). Flower buds with meiotic ceils