全文获取类型
收费全文 | 6631篇 |
免费 | 639篇 |
国内免费 | 3篇 |
专业分类
7273篇 |
出版年
2022年 | 58篇 |
2021年 | 128篇 |
2020年 | 69篇 |
2019年 | 73篇 |
2018年 | 90篇 |
2017年 | 67篇 |
2016年 | 145篇 |
2015年 | 292篇 |
2014年 | 283篇 |
2013年 | 418篇 |
2012年 | 519篇 |
2011年 | 487篇 |
2010年 | 278篇 |
2009年 | 235篇 |
2008年 | 404篇 |
2007年 | 412篇 |
2006年 | 350篇 |
2005年 | 304篇 |
2004年 | 311篇 |
2003年 | 295篇 |
2002年 | 289篇 |
2001年 | 80篇 |
2000年 | 56篇 |
1999年 | 78篇 |
1998年 | 94篇 |
1997年 | 58篇 |
1996年 | 58篇 |
1995年 | 49篇 |
1994年 | 60篇 |
1993年 | 47篇 |
1992年 | 43篇 |
1991年 | 42篇 |
1990年 | 50篇 |
1989年 | 36篇 |
1988年 | 37篇 |
1986年 | 36篇 |
1985年 | 42篇 |
1984年 | 42篇 |
1983年 | 48篇 |
1982年 | 54篇 |
1981年 | 43篇 |
1980年 | 38篇 |
1979年 | 30篇 |
1978年 | 30篇 |
1976年 | 35篇 |
1975年 | 30篇 |
1974年 | 31篇 |
1973年 | 33篇 |
1971年 | 29篇 |
1969年 | 28篇 |
排序方式: 共有7273条查询结果,搜索用时 15 毫秒
991.
Ruth H. Leeney Rachel Amies Annette C. Broderick Matthew J. Witt Jan Loveridge Joana Doyle Brendan J. Godley 《Biodiversity and Conservation》2008,17(10):2323-2338
Marine vertebrate strandings data can provide insights into the long-term dynamics of cetacean populations, and the threats
they face. We investigate whether the spatio-temporal patterns of cetacean strandings around Cornwall, SW Britain, have changed
in the past century. Analysis of strandings from 1911 to 2006 (n = 2,257) show that, since the mid-1970s, the relative frequency of strandings of common dolphins (Delphinus delphis), harbour porpoises (Phocoena phocoena) and pilot whales (Globicephala melas) has increased significantly. Seasonal peaks in strandings frequencies are apparent, between December and March for harbour
porpoises and common dolphins, and between November and January for pilot whales. There were significant positive trends in
the number of common dolphin and harbour porpoise strandings, as a proportion of total strandings, over time. Strandings of
common dolphins, porpoises and all other species occur more frequently on the south coast of Cornwall. A total of 415 cetaceans
were subject to full veterinary necropsy to determine cause of death, between 1990 and 2006, and 253 (61%) of these individuals
were determined to have died due to bycatch in fishing gear. Analyses of industrialised fishing pressure in UK waters show
the seas around Cornwall to be one of the most heavily fished areas of the UK. We suggest a number of factors that could be
responsible for the recent increases in cetacean strandings in southwest UK waters in recent years, including survey effort,
as well as abundance and range shifts that are potentially linked with climate change. Although detectable levels of bycatch
rate have not increased over time, fisheries interactions are in significant part responsible for mortality patterns and are
worthy of more detailed investigation.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
992.
Corte JR Fang T Pinto DJ Han W Hu Z Jiang XJ Li YL Gauuan JF Hadden M Orton D Rendina AR Luettgen JM Wong PC He K Morin PE Chang CH Cheney DL Knabb RM Wexler RR Lam PY 《Bioorganic & medicinal chemistry letters》2008,18(9):2845-2849
Introduction of the phenyl piperidinone and phenyl pyridinone P4 moieties in the anthranilamide scaffold led to potent, selective, and orally bioavailable inhibitors of factor Xa. Anthranilamide 28 displayed comparable efficacy to apixaban in the rabbit arteriovenous-shunt (AV) thrombosis model. 相似文献
993.
994.
New techniques and findings in the study of a candidate allelochemical implicated in invasion success 总被引:7,自引:1,他引:6
Amy C. Blair Bradley D. Hanson Galen R. Brunk Robin A. Marrs Philip Westra Scott J. Nissen Ruth A. Hufbauer 《Ecology letters》2005,8(10):1039-1047
Allelopathy has been hypothesized to promote the success of invasive plants. Support for the role of allelopathy in invasions has emerged from research on the candidate allelochemical (?)‐catechin, which is secreted by spotted knapweed. Here we describe new methods to quantify catechin in liquid and soil. With a new technique, we assayed catechin production by individual plants in liquid media and found levels up to two orders of magnitude less than previously reported. An acetone/water solution provided consistent recovery of catechin from soil, with percent recovery depending upon soil type. We evaluated soils from two spotted knapweed sites in Montana, USA, but found no measurable catechin. Idaho fescue, a native species reportedly sensitive to catechin, only exhibited slightly reduced growth at concentrations 10 times higher than previously reported to cause 100% mortality. Our results emphasize that more research is required to clarify the role of catechin in the invasion of spotted knapweed. 相似文献
995.
Peter J. Edmunds Ruth D. Gates William Leggat Ove Hoegh-Guldberg Laurie Allen-Requa 《Invertebrate Biology》2005,124(3):185-193
Abstract. Pre-settlement events play an important role in determining larval success in marine invertebrates with bentho-pelagic life histories, yet the consequences of these events typically are not well understood. The purpose of this study was to examine the pre-settlement impacts of different seawater temperatures on the size and population density of dinoflagellate symbionts in brooded larvae of the Caribbean coral Porites astreoides. Larvae were collected from P. astreoides at 14–20 m depth on Conch Reef (Florida) in June 2002, and incubated for 24 h at 15 temperatures spanning the range 25.1°–30.0°C in mean increments of 0.4±0.1°C (±SD). The most striking feature of the larval responses was the magnitude of change in both parameters across this 5°C temperature range within 24 h. In general, larvae were largest and had the highest population densities of Symbiodinium sp. between 26.4°–27.7°C, and were smallest and had the lowest population densities at 25.8°C and 28.8°C. Larval size and symbiont population density were elevated slightly (relative to the minimal values) at the temperature extremes of 25.1°C and 30°C. These data demonstrate that coral larvae are highly sensitive to seawater temperature during their pelagic phase, and respond through changes in size and the population densities of Symbiodinium sp. to ecologically relevant temperature signals within 24 h. The extent to which these changes are biologically meaningful will depend on the duration and frequency of exposure of coral larvae to spatio-temporal variability in seawater temperature, and whether the responses have cascading effects on larval success and their entry to the post-settlement and recruitment phase. 相似文献
996.
997.
A survey of Daldinia species with large ascospores 总被引:1,自引:1,他引:0
Marc Stadler Thomas Lsse Jack A. Simpson Hartmund Wollweber 《Mycological Research》2004,108(9):1025-1041
998.
OBJECTIVE: To compare cytomorphology preservation and immunohistochemistry results between conventional cell blocks (CCB) and cytoscrape cell blocks (SCB). STUDY DESIGN: Fine needle aspiration (FNAC) was done in 17 consecutive cases. Air-dried smears for May-Grünwald-Giemsa stain and wet-fixed smear for hematoxylin-eosin (H-E) stain were prepared. Simultaneously another pass was made in each case for preparation of material for CCB. One of the H-E-stained smears was spared for SCB. SCB was compared with CCB for cell morphology. Immunostaining was performed both cell blocks, as well as on FNA smears in 8 cases. Results were evaluated for intensity of staining and percentage of cells showing positivity. RESULTS: CCB and SCB sections showed adequate cellularity in all cases. Morphologic preservation was good in SCB sections. There was good architectural and nuclear preservation in all cases of SCB. Immunostaining results showed better and clear intensity of staining with little background in all cell block cases. CONCLUSION: SCB is a valuable technique in cell blocks from stained FNA smears. The cytomorphologic details are equally good in SCB and CCB. Additional panels of immunostaining can be done on SCB for better diagnosis and classification, particularly in cases in which repeat FNA is not possible. 相似文献
999.
The first fossil evidence for the fern genus Todea has been recovered from the Lower Cretaceous of British Columbia, Canada, providing paleontological data to strengthen hypotheses regarding patterns of evolution and phylogeny within Osmundaceae. The fossil consists of a branching rhizome, adventitious roots, and leaf bases. The dictyoxylic stem has up to eight xylem bundles around a sclerenchymatous pith. Leaf traces diverge from cauline bundles in a typical osmundaceous pattern and leaf bases display a sheath of sclerenchyma around a C-shaped xylem trace with 2-8 protoxylem strands. Within the adaxial concavity of each leaf trace, a single sclerenchyma bundle becomes C-shaped as it enters the cortex. The sclerotic cortex is heterogeneous with an indistinct outer margin. The discovery of Todea tidwellii sp. nov. reveals that the genus Todea evolved by the Lower Cretaceous. A phylogenetic analysis combining morphological characters of living and extinct species with a previously published nucleotide sequence matrix confirms the taxonomic placement of T. tidwellii. Results also support the hypothesis that Osmunda s.l. represents a paraphyletic assemblage and that living species be segregated into two genera, Osmunda and Osmundastrum. Fossil evidence confirms that Osmundaceae originated in the Southern Hemisphere during the Permian, underwent rapid diversification, and species extended around the world during the Triassic. Crown group Osmundaceae originated by the Late Triassic, with living species appearing by the Late Cretaceous. 相似文献
1000.
Detergents are commonly used for the extraction of hydrophobic proteins and must be removed for sensitive detection of peptides by mass spectrometry. We demonstrate that ethyl acetate is able to extract octylglycoside from a protease digest without loss of peptides or interference with the peptide mass spectral profile. Ethyl acetate extraction was also found to reduce interference by sodium dodecyl sulfate, Nonidet P-40, or Triton X-100 in the mass spectrometry analysis. 相似文献