全文获取类型
收费全文 | 2289篇 |
免费 | 266篇 |
出版年
2021年 | 16篇 |
2019年 | 18篇 |
2018年 | 24篇 |
2016年 | 37篇 |
2015年 | 71篇 |
2014年 | 65篇 |
2013年 | 90篇 |
2012年 | 103篇 |
2011年 | 96篇 |
2010年 | 52篇 |
2009年 | 55篇 |
2008年 | 100篇 |
2007年 | 99篇 |
2006年 | 90篇 |
2005年 | 84篇 |
2004年 | 99篇 |
2003年 | 69篇 |
2002年 | 75篇 |
2001年 | 88篇 |
2000年 | 63篇 |
1999年 | 57篇 |
1998年 | 37篇 |
1997年 | 36篇 |
1996年 | 25篇 |
1995年 | 26篇 |
1994年 | 23篇 |
1993年 | 22篇 |
1992年 | 39篇 |
1991年 | 33篇 |
1990年 | 55篇 |
1989年 | 48篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 39篇 |
1985年 | 36篇 |
1984年 | 30篇 |
1983年 | 29篇 |
1982年 | 20篇 |
1981年 | 19篇 |
1979年 | 29篇 |
1978年 | 30篇 |
1977年 | 20篇 |
1976年 | 32篇 |
1975年 | 24篇 |
1974年 | 23篇 |
1973年 | 27篇 |
1972年 | 24篇 |
1971年 | 17篇 |
1970年 | 21篇 |
1969年 | 15篇 |
排序方式: 共有2555条查询结果,搜索用时 15 毫秒
121.
122.
In the face of demands for researchers to engage more actively with a wider range of publics and to capture different kinds of research impacts and engagements, we explored the ways a small number of environmental researchers use traditional and social media to disseminate research. A questionnaire was developed to investigate the impact of different media as a tool to broker contact between researchers and a variety of different stakeholders (for example, publics, other researchers, policymakers, journalists) as well as how researchers perceive that their use of these media has changed over the past five years. The questionnaire was sent to 504 researchers whose work had featured in a policy-oriented e-news service. 149 valid responses were received (29%). Coverage in traditional media (newspapers, broadcast) not only brokers contact with other journalists, but is a good source of contact from other researchers (n=47, 62%) and members of the public (n=36, 26%). Although the use of social media was limited amongst our sample, it did broker contact with other researchers (n=17, 47%) and the public (n=10, 28%). Nevertheless, few environmental researchers were actively using social media to disseminate their research findings, with many continuing to rely on academic journals and face-to-face communication to reach both academic and public audiences. 相似文献
123.
Gabriel Rosser Alexander G. Fletcher David A. Wilkinson Jennifer A. de Beyer Christian A. Yates Judith P. Armitage Philip K. Maini Ruth E. Baker 《PLoS computational biology》2013,9(10)
Tracking bacteria using video microscopy is a powerful experimental approach to probe their motile behaviour. The trajectories obtained contain much information relating to the complex patterns of bacterial motility. However, methods for the quantitative analysis of such data are limited. Most swimming bacteria move in approximately straight lines, interspersed with random reorientation phases. It is therefore necessary to segment observed tracks into swimming and reorientation phases to extract useful statistics. We present novel robust analysis tools to discern these two phases in tracks. Our methods comprise a simple and effective protocol for removing spurious tracks from tracking datasets, followed by analysis based on a two-state hidden Markov model, taking advantage of the availability of mutant strains that exhibit swimming-only or reorientating-only motion to generate an empirical prior distribution. Using simulated tracks with varying levels of added noise, we validate our methods and compare them with an existing heuristic method. To our knowledge this is the first example of a systematic assessment of analysis methods in this field. The new methods are substantially more robust to noise and introduce less systematic bias than the heuristic method. We apply our methods to tracks obtained from the bacterial species Rhodobacter sphaeroides and Escherichia coli. Our results demonstrate that R. sphaeroides exhibits persistence over the course of a tumbling event, which is a novel result with important implications in the study of this and similar species. 相似文献
124.
Samantha Wilkinson Zen H. Lu Hendrik-Jan Megens Alan L. Archibald Chris Haley Ian J. Jackson Martien A. M. Groenen Richard P. M. A. Crooijmans Rob Ogden Pamela Wiener 《PLoS genetics》2013,9(4)
Following domestication, livestock breeds have experienced intense selection pressures for the development of desirable traits. This has resulted in a large diversity of breeds that display variation in many phenotypic traits, such as coat colour, muscle composition, early maturity, growth rate, body size, reproduction, and behaviour. To better understand the relationship between genomic composition and phenotypic diversity arising from breed development, the genomes of 13 traditional and commercial European pig breeds were scanned for signatures of diversifying selection using the Porcine60K SNP chip, applying a between-population (differentiation) approach. Signatures of diversifying selection between breeds were found in genomic regions associated with traits related to breed standard criteria, such as coat colour and ear morphology. Amino acid differences in the EDNRB gene appear to be associated with one of these signatures, and variation in the KITLG gene may be associated with another. Other selection signals were found in genomic regions including QTLs and genes associated with production traits such as reproduction, growth, and fat deposition. Some selection signatures were associated with regions showing evidence of introgression from Asian breeds. When the European breeds were compared with wild boar, genomic regions with high levels of differentiation harboured genes related to bone formation, growth, and fat deposition. 相似文献
125.
Xinzhong Li Andrew J. Buckton Samuel L. Wilkinson Shibu John Roddy Walsh Tomas Novotny Iveta Valaskova Manu Gupta Laurence Game Paul J R. Barton Stuart A. Cook James S. Ware 《PloS one》2013,8(7)
Background
Molecular genetic testing is recommended for diagnosis of inherited cardiac disease, to guide prognosis and treatment, but access is often limited by cost and availability. Recently introduced high-throughput bench-top DNA sequencing platforms have the potential to overcome these limitations.Methodology/Principal Findings
We evaluated two next-generation sequencing (NGS) platforms for molecular diagnostics. The protein-coding regions of six genes associated with inherited arrhythmia syndromes were amplified from 15 human samples using parallelised multiplex PCR (Access Array, Fluidigm), and sequenced on the MiSeq (Illumina) and Ion Torrent PGM (Life Technologies). Overall, 97.9% of the target was sequenced adequately for variant calling on the MiSeq, and 96.8% on the Ion Torrent PGM. Regions missed tended to be of high GC-content, and most were problematic for both platforms. Variant calling was assessed using 107 variants detected using Sanger sequencing: within adequately sequenced regions, variant calling on both platforms was highly accurate (Sensitivity: MiSeq 100%, PGM 99.1%. Positive predictive value: MiSeq 95.9%, PGM 95.5%). At the time of the study the Ion Torrent PGM had a lower capital cost and individual runs were cheaper and faster. The MiSeq had a higher capacity (requiring fewer runs), with reduced hands-on time and simpler laboratory workflows. Both provide significant cost and time savings over conventional methods, even allowing for adjunct Sanger sequencing to validate findings and sequence exons missed by NGS.Conclusions/Significance
MiSeq and Ion Torrent PGM both provide accurate variant detection as part of a PCR-based molecular diagnostic workflow, and provide alternative platforms for molecular diagnosis of inherited cardiac conditions. Though there were performance differences at this throughput, platforms differed primarily in terms of cost, scalability, protocol stability and ease of use. Compared with current molecular genetic diagnostic tests for inherited cardiac arrhythmias, these NGS approaches are faster, less expensive, and yet more comprehensive. 相似文献126.
Alexander Jason B. Bunce Michael White Nicole Wilkinson Shaun P. Adam Arne A. S. Berry Tina Stat Michael Thomas Luke Newman Stephen J. Dugal Laurence Richards Zoe T. 《Coral reefs (Online)》2020,39(1):159-171
Coral Reefs - Cumulative anthropogenic pressures have triggered a global decline in the health of marine ecosystems, and coral reefs, in particular, are in crisis. With climate and... 相似文献
127.
Abstract Post-construction avifauna investigations were undertaken at Project West Wind, Meridian Energy Limited's 62-turbine wind farm on the Wellington south coast. These investigations were required in accordance with the resource consent conditions to quantify the level of avian mortalities occurring at the wind farm, particularly in regard to New Zealand falcon (Falco novaeseelandiae), kākā (Nestor meridionalis) and kererū (Hemiphaga novaeseelandiae). This is the first comprehensive study at a New Zealand operating wind farm. The methods included three field components necessary to calculate annual estimates of mortalities across the wind farm site: routine turbine searches; carcass detection trials; and carcass removal trials. Results from years 1 and 2 of a three-year programme are presented. To date, mortalities have been recorded for 17 taxa at 18 of the 24 study turbines. There have been no recorded mortalities of falcon, kākā or kererū. Australasian harrier (Circus approximans) has been the species for which the most mortalities have been recorded. Overall estimated annual mortality rates for years 1 and 2 were calculated to be approximately six and five birds per turbine respectively. 相似文献
128.
129.
130.
CapsuleA second full survey in which we estimate the population at 1073 birds (95% CL 549–2041). Aims To provide an up-to-date estimate of the population size of Capercaillie in Britain and to identify habitat use by Capercaillie in winter. Methods Using the census technique of distance sampling, flushed birds were counted along line transects walked in stratified random blocks of forest within the species' range. The same transects were surveyed as used in the 1992–94 survey, enabling direct comparison of the results. Densities were estimated using the DISTANCE program, combining these data with those from a similar survey of Capercaillie in 1992–94 to allow a more robust estimate of the detection function. Results The Capercaillie population was estimated to be 1073 birds (95% CL 549–2041). This represents a decline of 51% between the two surveys, at a rate of 13% per annum. Females declined at a faster rate than males resulting in a marked change in the sex-ratio, which was close to 1:1 for the entire population. Bird densities declined by a similar magnitude in both native pinewoods and other woodlands in comparison to 1992–94. Even so, densities were significantly higher in native pinewoods (1.63 km-2) than in other woodlands (0.43 km-2), which comprised largely planted Scots Pine Pinus sylvestris, larch Larix spp., Lodgepole Pine Pinus contorta and Sitka Spruce Picea sitchensis. Overall, Capercaillie tended to use forest stands with a high proportion of Scots Pine and Blaeberry Vaccinium myrtilus; features that are typical of native pinewoods and old pine plantations. Conclusions The Capercaillie population has undergone a dramatic decline during the last five years. The decline has been greater for females and has occurred across habitats. This information is helping to focus efforts in securing the future for Capercaillie in Scotland, which has now become a major conservation challenge. 相似文献