全文获取类型
收费全文 | 137840篇 |
免费 | 4513篇 |
国内免费 | 842篇 |
出版年
2021年 | 632篇 |
2020年 | 412篇 |
2019年 | 517篇 |
2018年 | 12379篇 |
2017年 | 11121篇 |
2016年 | 8422篇 |
2015年 | 2410篇 |
2014年 | 2229篇 |
2013年 | 2856篇 |
2012年 | 7258篇 |
2011年 | 15803篇 |
2010年 | 13830篇 |
2009年 | 9851篇 |
2008年 | 12237篇 |
2007年 | 13775篇 |
2006年 | 2620篇 |
2005年 | 2927篇 |
2004年 | 3320篇 |
2003年 | 3281篇 |
2002年 | 2968篇 |
2001年 | 844篇 |
2000年 | 663篇 |
1999年 | 616篇 |
1998年 | 652篇 |
1997年 | 523篇 |
1996年 | 472篇 |
1995年 | 426篇 |
1994年 | 401篇 |
1993年 | 425篇 |
1992年 | 487篇 |
1991年 | 450篇 |
1990年 | 366篇 |
1989年 | 347篇 |
1988年 | 328篇 |
1987年 | 322篇 |
1986年 | 292篇 |
1985年 | 380篇 |
1984年 | 379篇 |
1983年 | 326篇 |
1982年 | 397篇 |
1981年 | 311篇 |
1980年 | 304篇 |
1979年 | 237篇 |
1978年 | 261篇 |
1977年 | 246篇 |
1975年 | 214篇 |
1974年 | 223篇 |
1973年 | 234篇 |
1972年 | 404篇 |
1971年 | 402篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
41.
W W Johnston 《Acta cytologica》1984,28(3):218-224
From the time of its introduction, in 1973, through November 1983, fine needle aspiration biopsy ( FNAB ) of the lung was used in the diagnostic evaluation of 1,015 patients at the Duke University Medical Center and Durham Veterans Administration Medical Center. A diagnosis of a primary lung cancer was established in 49.4% of the cases and malignant neoplasm metastatic to the lung in 14.1%; a benign diagnosis was made in 31.1% and an inconclusive one in 5.4%. In 123 patients in whom lung tissue was also available, cancer was correctly diagnosed by FNAB in 83.8%, not diagnosed by FNAB in 14.6% and incorrectly diagnosed in 1.6%. In 37 patients without cancer, FNAB detected an infectious organism or a specific morphologic type of inflammation (abscess and granuloma). 相似文献
42.
The ability of an invasive plant to occupy new areas is often attributed to both morphological and physiological plasticities
that allow them to remain viable over a wide range of environmental conditions. Studies addressing the ecological requirements
of Microstegium vimineum often consider soil moisture or soil moisture along with other factors as important explanatory components for the establishment
and persistence of this invasive monocot. However, controlled studies specifically targeting water relations in M. vimineum are needed. Therefore, the purpose of this study was to determine how different water availabilities influence the growth
and physiological performance of M. vimineum. This study utilized experimental microcosms to achieve different water availabilities including low soil moisture (<15%
water), moderate soil moisture (ca. 20–30%), and flooded conditions. While both flooded and low soil moisture resulted in
diminished growth, M. vimineum still survived under these conditions. Physiological processes including C4 metabolism, minimum stress under low water conditions, and the ability to increase tissue rigidity may confer some advantages
to M. vimineum during periods of limiting water conditions. Similarly, the proportionally low root biomass, shallow root structure, and
its ability to maintain stable water relations during flooding and/or soil waterlogging may facilitate M. vimineum’s ability to invade mesic habitats. It is likely, therefore, that the capacity to tolerate both low soil moistures and flooded
conditions has enhanced the ability of M. vimineum to populate disturbed systems in central North Carolina. 相似文献
43.
Paul D. Bonnitcha Simon R. Bayly Mark B.M. Theobald Helen M. Betts Jason S. Lewis Jonathan R. Dilworth 《Journal of inorganic biochemistry》2010,104(2):126-9888
Combination agents comprising two different pharmacophores with the same biological target have the potential to show additive or synergistic activity. Bis(thiosemicarbazonato)copper(II) complexes (e.g. 64Cu-ATSM) and nitroimidazoles (e.g. 18F-MISO) are classes of tracer used for the delineation of tumor hypoxia by positron emission tomography (PET). Three nitroimidazole-bis(thiosemicarbazonato)copper(II) conjugates were produced in order to investigate their potential as combination hypoxia imaging agents. Two were derived from the known bifunctional bis(thiosemicarbazone) H2ATSM/A and the third from the new precursor diacetyl-2-(4-N-methyl-3-thiosemicarbazone)-3-(4-N-ethylamino-3-thiosemicarbazone) - H2ATSM/en. Oxygen-dependent uptake studies were performed using the 64Cu radiolabelled complexes in EMT6 carcinoma cells. All the complexes displayed appreciable hypoxia selectivity, with the nitroimidazole conjugates displaying greater selectivity than a simple propyl derivative used as a control. Participation of the nitroimidazole group in the trapping mechanism is indicated by the increased hypoxic uptake of the 2- vs. the 4-substituted 64Cu-ATSM/A derivatives. The 2-nitroimidazole derivative of 64Cu-ATSM/en demonstrated superior hypoxia selectivity to 64Cu-ATSM over the range of oxygen concentrations tested. Biodistribution of the radiolabelled 2-nitroimidazole conjugates was carried out in EMT6 tumor-bearing mice. The complexes showed significantly different uptake trends in comparison to each other and previously studied Cu-ATSM derivatives. Uptake of the Cu-ATSM/en conjugate in non-target organs was considerably lower than for derivatives based on Cu-ATSM/A. 相似文献
44.
A particularly vexing phenomenon within invasion ecology is the occurrence of spontaneous collapses within seemingly well-established exotic populations. Here, we assess the frequency of collapses among 68 exotic bird populations established in Hawaii, Puerto Rico, Los Angeles and Miami. Following other published definitions, we define a ‘collapse’ as a decline in abundance of ≥90 % within ≤10 years that lasts for at least 3 years. We show that 44 of the 68 exotic bird populations have exhibited declines at some point within their time series. Sixteen of the populations declined sufficiently to be defined as collapsed. It took on average 3.8 ± 1.8 years for populations to decline into a collapsed state, and this state persisted on average for 7.1 ± 6.3 years across (collapsed) populations. We compared the severity and duration of declines across all 44 declining populations according to taxonomic Order and geographic region. Neither variable explained substantial variation in the metrics of collapse. Our results indicate that severe, rapid, and persistent population declines may be common among exotic populations. We suggest that incorporating the probability and persistence of collapses into management decisions can inform efforts to enact control or eradication measures. We also suggest that applying our approach to other taxa and locations is crucial for improving our understanding of when and where collapses are likely to occur. 相似文献
45.
46.
47.
48.
David Skelly 《BMC biology》2010,8(1):136
A new study of divergence in freshwater fish provides strong evidence of rapid, temperature-mediated adaptation. This study
is particularly important in the ongoing debate over the extent and significance of evolutionary response to climate change
because divergence has occurred in relatively few generations in spite of ongoing gene flow and in the aftermath of a significant
genetic bottleneck, factors that have previously been considered obstacles to evolution. Climate change may thus be more likely
to foster contemporary evolutionary responses than has been anticipated, and I argue here for the importance of investigating
their possible occurrence. 相似文献
49.
Abstract— 2-Hydroxy-, 2-chloro-, 2- and Cmethyl-GABA are linear competitive inhibitors of GABA uptake in rat brain slices. These analogues are thus potential substrates for the GABA transport system and possible'false transmitters'. 2-Hydroxy-GABA is the most potent inhibitor of GABA uptake yet described. No specific inhibitor of GABA uptake was revealed amongst the drugs tested. 相似文献
50.