全文获取类型
收费全文 | 9356篇 |
免费 | 1043篇 |
国内免费 | 3篇 |
专业分类
10402篇 |
出版年
2022年 | 84篇 |
2021年 | 126篇 |
2020年 | 92篇 |
2019年 | 106篇 |
2018年 | 120篇 |
2017年 | 152篇 |
2016年 | 198篇 |
2015年 | 332篇 |
2014年 | 346篇 |
2013年 | 455篇 |
2012年 | 592篇 |
2011年 | 610篇 |
2010年 | 373篇 |
2009年 | 353篇 |
2008年 | 513篇 |
2007年 | 504篇 |
2006年 | 469篇 |
2005年 | 447篇 |
2004年 | 394篇 |
2003年 | 379篇 |
2002年 | 380篇 |
2001年 | 162篇 |
2000年 | 163篇 |
1999年 | 162篇 |
1998年 | 150篇 |
1997年 | 105篇 |
1996年 | 103篇 |
1995年 | 96篇 |
1994年 | 75篇 |
1993年 | 88篇 |
1992年 | 117篇 |
1991年 | 84篇 |
1990年 | 91篇 |
1989年 | 101篇 |
1988年 | 96篇 |
1987年 | 96篇 |
1986年 | 75篇 |
1985年 | 88篇 |
1984年 | 82篇 |
1983年 | 75篇 |
1982年 | 85篇 |
1981年 | 75篇 |
1980年 | 73篇 |
1979年 | 81篇 |
1978年 | 86篇 |
1977年 | 72篇 |
1976年 | 55篇 |
1975年 | 64篇 |
1974年 | 70篇 |
1973年 | 56篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Sex ratios under asymmetrical local mate competition in the parasitoid wasp Nasonia vitripennis 总被引:1,自引:0,他引:1
Sex ratio theory has proved remarkably useful in testing theadaptive nature of animal behavior. A particularly productivearea in this respect is Hamilton's theory of local mate competition(LMC), which has been extended in numerous directions to includegreater biological realism, allowing more detailed tests inspecific organisms. We have presented one such extension, termedasymmetrical LMC, which occurs when egg laying by females ona patch is asynchronous, and emerging males do not disperse,resulting in the extent of LMC on a patch varying over time.Our aim here is to test whether the parasitoid wasp Nasoniavitripennis responds to variation in the degree of asymmetricalLMC. Specifically, we show that females adjust their offspringsex ratios in response to (1) variation in the amount of asynchronyin emergence between broods on a patch and (2) the number andproportion of previously parasitized hosts on the patch. Ourresults provide qualitative support for the predictions of theory,suggesting new levels of complexity in the sex ratio behaviorof this much-studied organism. However, our results do not alwaysprovide quantitative support for theory, suggesting furthercomplexities that must be clarified. 相似文献
2.
Isoleucine and Valine Metabolism in Escherichia coli XVIII. Induction of Acetohydroxy Acid Isomeroreductase 总被引:7,自引:7,他引:0 下载免费PDF全文
The regulation by substrate induction of the acetohydroxy acid isomeroreductase was studied in Escherichia coli. Induction was inhibited by chloramphenicol and rifampin. The addition of rifampin resulted in a decay of the capacity to form isomeroreductase. This was attributed to the breakdown of the isomeroreductase messenger, which had a half-life of about 45 sec at 37 C. Induction of isomeroreductase was enhanced by including glucose in the medium. This effect was shown to be due in part to the lowering of the pH of the medium, which presumably made inducer entry more rapid. 相似文献
3.
An Extreme Value Analysis of Pollutant Concentrations in Surface Soils Due to Atmospheric Deposition
Risk assessments often rely on deterministic models using long-term averages or “steady-state” values of input variables. Such models do not provide the information needed to estimate acute exposures. This study uses extreme value theory to examine the frequency and magnitude of daily pollutant concentrations in surface soils predicted at six U.S. locations. Concentrations are predicted using a deposition-leaching model and 50 years of historical precipitation data. A stochastic model also is used to generate 1000 years of precipitation data as modeling inputs for each location. The annual maximum concentrations at each site are fitted to a Gumbel type I distribution to estimate occurrence probability. For soluble pollutants, the predicted concentration varied substantially with precipitation, and the maximum daily concentrations exceeded annual averages by 4 to 8 times. Observed and synthetic precipitation data produced similar results at most study locations, though the synthetic data provided a slightly better fit to the Gumbel type I distribution. The precipitation model allows the generation of representative precipitation data that extend limited historical records. The extreme value analysis facilitates the evaluation of maximum pollutant concentrations, return periods, and other statistics that are important in evaluating acute exposures. 相似文献
4.
5.
6.
Polypeptide components of an excitable plasma membrane 总被引:4,自引:0,他引:4
7.
8.
9.
J. Stuart M. H. Winterborn R. H. R. White R. M. Flinn 《BMJ (Clinical research ed.)》1974,3(5925):217-221
The treatment of five children with the haemolytic-uraemic syndrome using streptokinase is described to illustrate the difficulties and limitations of thrombolytic therapy in this disease. This experience is germane to the design of multicentre clinical trials.A multivariate analysis relating clinical outcome to the data obtained at the time of admission was also carried out for 31 children with the disease treated in four centres. The results suggest that this technique may help to identify those patients likely to have a fatal outcome. An expanded form of this type of analysis should be incorporated in future clinical trials. 相似文献
10.
J. Stuart Horner 《BMJ (Clinical research ed.)》1974,3(5923):100-103
Almost 950,000 of the 21 million passengers passing through London (Heathrow) Airport in 1973 were seen by the health control unit, which is run by the London Borough of Hillingdon. The unit provides 24-hour medical cover and its responsibilities include x-ray examination for tuberculosis and screening passengers from smallpox-infected areas. It is suggested that, in view of changing epidemiological patterns throughout the world, there is a need to modify existing procedures rather than to abandon them. The development of a follow-up system for tracing passengers at risk and improvements in presenting information about health risks to intending travellers are advocated. While such proposals might be opposed, they could be practicable. 相似文献