全文获取类型
收费全文 | 85281篇 |
免费 | 2183篇 |
国内免费 | 47篇 |
出版年
2018年 | 962篇 |
2017年 | 1148篇 |
2016年 | 2889篇 |
2015年 | 6275篇 |
2014年 | 5851篇 |
2013年 | 5641篇 |
2012年 | 4829篇 |
2011年 | 2091篇 |
2010年 | 2106篇 |
2009年 | 2071篇 |
2008年 | 633篇 |
2007年 | 555篇 |
2006年 | 600篇 |
2005年 | 6689篇 |
2004年 | 5373篇 |
2003年 | 3595篇 |
2002年 | 1153篇 |
2001年 | 1124篇 |
2000年 | 348篇 |
1999年 | 1515篇 |
1998年 | 358篇 |
1997年 | 187篇 |
1992年 | 1935篇 |
1991年 | 2022篇 |
1990年 | 2089篇 |
1989年 | 2008篇 |
1988年 | 1966篇 |
1987年 | 1818篇 |
1986年 | 1633篇 |
1985年 | 1655篇 |
1984年 | 1092篇 |
1983年 | 839篇 |
1982年 | 469篇 |
1981年 | 430篇 |
1980年 | 371篇 |
1979年 | 1079篇 |
1978年 | 762篇 |
1977年 | 612篇 |
1976年 | 637篇 |
1975年 | 877篇 |
1974年 | 1006篇 |
1973年 | 1010篇 |
1972年 | 961篇 |
1971年 | 930篇 |
1970年 | 823篇 |
1969年 | 832篇 |
1968年 | 737篇 |
1967年 | 753篇 |
1966年 | 585篇 |
1965年 | 432篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
102.
103.
Valeria L. Martin-Albarracin Guillermo C. Amico Daniel Simberloff Martin A. Nu?ez 《PloS one》2015,10(11)
Introduction and naturalization of non-native species is one of the most important threats to global biodiversity. Birds have been widely introduced worldwide, but their impacts on populations, communities, and ecosystems have not received as much attention as those of other groups. This work is a global synthesis of the impact of nonnative birds on native ecosystems to determine (1) what groups, impacts, and locations have been best studied; (2) which taxonomic groups and which impacts have greatest effects on ecosystems, (3) how important are bird impacts at the community and ecosystem levels, and (4) what are the known benefits of nonnative birds to natural ecosystems. We conducted an extensive literature search that yielded 148 articles covering 39 species belonging to 18 families -18% of all known naturalized species. Studies were classified according to where they were conducted: Africa, Asia, Australasia, Europe, North America, South America, Islands of the Indian, of the Pacific, and of the Atlantic Ocean. Seven types of impact on native ecosystems were evaluated: competition, disease transmission, chemical, physical, or structural impact on ecosystem, grazing/ herbivory/ browsing, hybridization, predation, and interaction with other non-native species. Hybridization and disease transmission were the most important impacts, affecting the population and community levels. Ecosystem-level impacts, such as structural and chemical impacts were detected. Seven species were found to have positive impacts aside from negative ones. We provide suggestions for future studies focused on mechanisms of impact, regions, and understudied taxonomic groups. 相似文献
104.
105.
106.
Lorenzo Dal Bo Paolo Mazzucchelli Antonio Marzo 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2000,749(2):258
Zofenopril is a pro-drug designed to undergo metabolic hydrolysis yielding the active free sulfhydryl compound zofenoprilat, which is an angiotensin converting enzyme (ACE) inhibitor, endowed also with a marked cardioprotective activity. A simple, highly sensitive specific LC–MS–MS method was developed for the determination of zofenopril and zofenoprilat in human plasma. In order to prevent oxidative degradation of zofenoprilat and its internal standard, their free sulfhydryl groups were protected by treatment with N-ethylmaleimide (NEM), which produced the succinimide derivatives. The compounds and their corresponding fluorine derivatives, used as internal standards, were extracted from plasma with toluene. The reconstituted dried extracts were chromatographed and then monitored by a triple-stage-quadrupole instrument operating in the negative ion spray ionization mode. The method was validated over the concentration range of 1–300 ng/ml for zofenopril and 2–600 ng/ml for zofenoprilat. Inter- and intra-assay precision and accuracy of both zofenopril and zofenoprilat were better than 10%. The limit of quantitation was 1 ng/ml with zofenopril and 2 ng/ml with zofenoprilat. Extraction recovery proved to be on average 84.8% with zofenopril and 70.1% with zofenoprilat. Similar recoveries were shown by the above two internal standards. The method was applied to measure plasma concentrations of zofenopril and zofenoprilat in 18 healthy volunteers treated orally with zofenopril calcium salt at the dose of 60 mg. 相似文献
107.
Matthias Schlichting Pamela Menegazzi Charlotte Helfrich-F?rster 《Proceedings. Biological sciences / The Royal Society》2015,282(1815)
Circadian clocks are thought to be essential for timing the daily activity of animals, and consequently increase fitness. This view was recently challenged for clock-less fruit flies and mice that exhibited astonishingly normal activity rhythms under outdoor conditions. Compensatory mechanisms appear to enable even clock mutants to live a normal life in nature. Here, we show that gradual daily increases/decreases of light in the laboratory suffice to provoke normally timed sharp morning (M) and evening (E) activity peaks in clock-less flies. We also show that the compound eyes, but not Cryptochrome (CRY), mediate the precise timing of M and E peaks under natural-like conditions, as CRY-less flies do and eyeless flies do not show these sharp peaks independently of a functional clock. Nevertheless, the circadian clock appears critical for anticipating dusk, as well as for inhibiting sharp activity peaks during midnight. Clock-less flies only increase E activity after dusk and not before the beginning of dusk, and respond strongly to twilight exposure in the middle of the night. Furthermore, the circadian clock responds to natural-like light cycles, by slightly broadening Timeless (TIM) abundance in the clock neurons, and this effect is mediated by CRY. 相似文献
108.
Jan Suchá?ek Petr Sed’a Václav Friedrich Renata Wachowiak-Smolíková Mark P. Wachowiak 《PloS one》2016,11(11)
Media, and particularly TV media, have a great impact on the general public. In recent years, spatial patterns of information and the relevance of intangible geographies have become increasingly important. Gatekeeping plays a critical role in the selection of information that is transformed into media. Therefore, gatekeeping, through national media, also co-forms the generation of mental maps. In this paper, correspondence analysis (a statistical method) combined with cloud lines (a new visual analytics technique) is used to analyze how individual major regional events in one of the post-communist countries, the Czech Republic, penetrate into the media on a national scale. Although national news should minimize distortions about regions, this assumption has not been verified by our research. Impressions presented by the media of selected regions that were markedly influenced by one or several events in those regions demonstrate that gatekeepers, especially news reporters, functioned as a filter by selecting only a few specific, and in many cases, unusual events for dissemination. 相似文献
109.
110.