Silver and other metals in some aquatic bryophytes from streams in the lead mining district of Mid-Wales,Great Britain     

K. C. Jones  P. J. Peterson  B. E. Davies 《Water, air, and soil pollution》1985,24(3):329-338

A reconnaissance survey was made of trace metals in surface waters and three aquatic bryophytes (Scapania undulata,Hygrohypnum luridum andPolytrichum commune) in the mid-Wales Ag-Pb mining district. This area is contaminated by heavy metals from past mining. Elevated levels of Ag, Cd, Cu, Pb, and Zn were found in water and plant tissue. All the plants accumulated Ag and the other metals butScapania appears the most suitable as a plant monitor of Ag-polluted waters.  相似文献   

The re-expansion and improving status of the silver-spotted skipper butterfly (Hesperia comma) in Britain: a metapopulation success story     

Zoe G. Davies  Robert J. Wilson  Tom M. Brereton 《Biological conservation》2005,124(2):189-198

Many specialist species are declining as a result of habitat loss and fragmentation, such that conservation actions typically aim to stem rates of decline rather than bring about genuine recovery. Here, we document the recovery of a species from former population refuges. An extensive survey of the entire British range of Hesperia comma, conducted in 2000, recorded over three times the number of tetrads (2 km × 2 km grid squares) occupied in 1982. This was accompanied by a fourfold increase in the number of populations and a 10-fold increase in the habitat area occupied. The improving status of H. comma is the product of good habitat management, recovering rabbit populations and climate warming, which have improved the quality, and increased the availability, of suitable habitat. This has enabled remnant metapopulations to expand, via distance-dependent colonisation, through large networks of habitat. Metapopulation recovery in H. comma demonstrates that landscape-scale conservation can be successful.  相似文献   

HST and VLT investigations of the fragments of comet C/1999 S4 (LINEAR)     

Weaver HA  Sekanina Z  Toth I  Delahodde CE  Hainaut OR  Lamy PL  Bauer JM  A'Hearn MF  Arpigny C  Combi MR  Davies JK  Feldman PD  Festou MC  Hook R  Jorda L  Keesey MS  Lisse CM  Marsden BG  Meech KJ  Tozzi GP  West R 《Science (New York, N.Y.)》2001,292(5520):1329-1333

At least 16 fragments were detected in images of comet C/1999 S4 (LINEAR) taken on 5 August 2000 with the Hubble Space Telescope (HST) and on 6 August with the Very Large Telescope (VLT). Photometric analysis of the fragments indicates that the largest ones have effective spherical diameters of about 100 meters, which implies that the total mass in the observed fragments was about 2 x 10(9) kilograms. The comet's dust tail, which was the most prominent optical feature in August, was produced during a major fragmentation event, whose activity peaked on UT 22.8 +/- 0.2 July 2000. The mass of small particles (diameters less than about 230 micrometers) in the tail was about 4 x 10(8) kilograms, which is comparable to the mass contained in a large fragment and to the total mass lost from water sublimation after 21 July 2000 (about 3 x 10(8) kilograms). HST spectroscopic observations during 5 and 6 July 2000 demonstrate that the nucleus contained little carbon monoxide ice (ratio of carbon monoxide to water is less than or equal to 0.4%), which suggests that this volatile species did not play a role in the fragmentation of C/1999 S4 (LINEAR).  相似文献   

Comparative Earth History and Late Permian Mass Extinction   总被引：6，自引：0，他引：6  

AH Knoll  RK Bambach  DE Canfield  JP Grotzinger 《Science (New York, N.Y.)》1996,273(5274):452-457

The repeated association during the late Neoproterozoic Era of large carbon-isotopic excursions, continental glaciation, and stratigraphically anomalous carbonate precipitation provides a framework for interpreting the reprise of these conditions on the Late Permian Earth. A paleoceanographic model that was developed to explain these stratigraphically linked phenomena suggests that the overturn of anoxic deep oceans during the Late Permian introduced high concentrations of carbon dioxide into surficial environments. The predicted physiological and climatic consequences for marine and terrestrial organisms are in good accord with the observed timing and selectivity of Late Permian mass extinction.  相似文献   

Transmissible multiple drug resistance in Enterobacteriaceae   总被引：33，自引：0，他引：33  

J E Davies  R Rownd 《Science (New York, N.Y.)》1972,176(36):758-768

  相似文献   

Planetary contamination I: the problem and the agreements     

Horowitz NH  Sharp RP  Davies RW 《Science (New York, N.Y.)》1967,155(3769):1501-1505

The sterility requirements for landed spacecraft tentatively adopted in the COSPAR resolution of 1964 are so severe as to pose a major obstacle to planetary exploration. This by itself would not justify modification of the re quirements, since preservation of the biological integrity of Mars is essential for proper exploration of the planet. However, when the physical and biological assumptions underlying the COSPAR recommendations are com pared with actual conditions on Mars, as established by recent observations, it becomes apparent that the COSPAR as sumptions are unrealistic in important respects. Specifically, the belief that eolian erosion on Mars can effect the release of spores trapped in the interior of solids in periods of time that are short compared with the time scale of the unmanned space program is unsup ported by either observation or theory. On the contrary, the analysis suggests that rates of eolian erosion on Mars are very low. Similarly, present knowledge of the Martian environment opposes the view that terrestrial microorganisms would readily contaminate the planet. The combination of dryness, lack of oxygen, and high ultraviolet flux makes the surface of Mars peculiarly unsuit able for the multiplication of terrestrial organisms. Recent studies give little sup port to the proposal that significant areas of geothermal activity exist on Mars. These various findings suggest that the COSPAR-recommended constraints could be substantially relaxed without compromising to any significant degree the biological condition of Mars. In particular, a distinction needs to be made between microorganisms trapped in solids and those on exposed sur faces of landed spacecraft. Surface sterility is an unconditional require ment, in the sense that it is imposed by considerations unrelated to the nature of the Martian environment. Sterilization of the interior of solids to the extreme level recommended by COSPAR, however, is based on the as sumption that entrapped organisms con stitute a substantial hazard to the ecology of Mars. This assumption now seems unjustified, and the need for a high degree of interior sterility is doubt ful. Current spacecraft-sterilization pol icies should be revised accordingly.  相似文献   

A High-Temperature Electrical Conduction Mechanism in the Lower Mantle Phase (Mg,Fe)1-xO     

DP Dobson  NC Richmond  JP Brodholt 《Science (New York, N.Y.)》1997,275(5307):1779-1781

Measurements of electrical conductivity at high pressure and temperature were taken on the lower mantle phase magnesiowustite with varying Fe3+ content. Although previous measurements at atmospheric pressure suggest Fe2+-Fe3+ hopping (small polaron) as the dominant conductivity mechanism, the present experiments show a change in charge transport mechanism with temperature. The lower temperature measurements are consistent with small polaron conduction, but at higher temperatures, which are more applicable to the lower mantle, a large polaron mechanism is suggested. Because these mechanisms have different temperature and compositional dependencies, this transition has important implications for extrapolation to mantle conditions.  相似文献   

Collagen synthesis by cells synchronously replicating DNA     

Davies LM  Priest JH  Priest RE 《Science (New York, N.Y.)》1968,159(3810):91-93

Replication and the performance of a differentiated function have been considered antagonistic processes. When cells in culture are partially tially synchronized with 5-fluoro-2'-deoxyuridine (FUdR), the synthesis of the specialized protein (collagen) is not reduced during chromosomal replication (S period). Collagen synthesis varies with general protein synthesis through the S period.  相似文献   

Wave propagation: the propagation of radio waves     

Davies K 《Science (New York, N.Y.)》1986,232(4756):1448

  相似文献   

Muscle research     

Davies RE 《Science (New York, N.Y.)》1977,197(4306):858

  相似文献