全文获取类型
收费全文 | 128424篇 |
免费 | 3442篇 |
国内免费 | 830篇 |
专业分类
132696篇 |
出版年
2023年 | 114篇 |
2022年 | 326篇 |
2021年 | 607篇 |
2020年 | 363篇 |
2019年 | 488篇 |
2018年 | 12332篇 |
2017年 | 11084篇 |
2016年 | 8308篇 |
2015年 | 2157篇 |
2014年 | 1985篇 |
2013年 | 2380篇 |
2012年 | 6992篇 |
2011年 | 15534篇 |
2010年 | 13602篇 |
2009年 | 9804篇 |
2008年 | 12036篇 |
2007年 | 13617篇 |
2006年 | 2525篇 |
2005年 | 2627篇 |
2004年 | 3055篇 |
2003年 | 2942篇 |
2002年 | 2682篇 |
2001年 | 701篇 |
2000年 | 455篇 |
1999年 | 454篇 |
1998年 | 493篇 |
1997年 | 369篇 |
1996年 | 336篇 |
1995年 | 285篇 |
1994年 | 256篇 |
1993年 | 291篇 |
1992年 | 265篇 |
1991年 | 238篇 |
1990年 | 177篇 |
1989年 | 189篇 |
1988年 | 169篇 |
1987年 | 147篇 |
1986年 | 115篇 |
1985年 | 149篇 |
1984年 | 175篇 |
1983年 | 146篇 |
1982年 | 148篇 |
1981年 | 121篇 |
1980年 | 112篇 |
1979年 | 74篇 |
1978年 | 79篇 |
1977年 | 76篇 |
1976年 | 54篇 |
1972年 | 276篇 |
1971年 | 296篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
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
32.
Virginia Edgcomb William Orsi John Bunge Sunok Jeon Richard Christen Chesley Leslin Mark Holder Gordon T Taylor Paula Suarez Ramon Varela Slava Epstein 《The ISME journal》2011,5(8):1344-1356
Microbial diversity and distribution are topics of intensive research. In two companion papers in this issue, we describe the results of the Cariaco Microbial Observatory (Caribbean Sea, Venezuela). The Basin contains the largest body of marine anoxic water, and presents an opportunity to study protistan communities across biogeochemical gradients. In the first paper, we survey 18S ribosomal RNA (rRNA) gene sequence diversity using both Sanger- and pyrosequencing-based approaches, employing multiple PCR primers, and state-of-the-art statistical analyses to estimate microbial richness missed by the survey. Sampling the Basin at three stations, in two seasons, and at four depths with distinct biogeochemical regimes, we obtained the largest, and arguably the least biased collection of over 6000 nearly full-length protistan rRNA gene sequences from a given oceanographic regime to date, and over 80 000 pyrosequencing tags. These represent all major and many minor protistan taxa, at frequencies globally similar between the two sequence collections. This large data set provided, via the recently developed parametric modeling, the first statistically sound prediction of the total size of protistan richness in a large and varied environment, such as the Cariaco Basin: over 36 000 species, defined as almost full-length 18S rRNA gene sequence clusters sharing over 99% sequence homology. This richness is a small fraction of the grand total of known protists (over 100 000–500 000 species), suggesting a degree of protistan endemism. 相似文献
33.
34.
35.
36.
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
37.
Li Zhang Mark Morrison Páraic ó Cuív Paul Evans Claire M. Rickard 《Journal of bacteriology》2012,194(23):6639
In recent years, Staphylococcus epidermidis has become a major nosocomial pathogen and the most common cause of intravascular catheter-related bacteremia, which can increase morbidity and mortality and significantly affect patient recovery. We report a draft genome sequence of Staphylococcus epidermidis AU12-03, isolated from an intravascular catheter tip. 相似文献
38.
Luke L. M. Heaton Eduardo López Philip K. Maini Mark D. Fricker Nick S. Jones 《Proceedings. Biological sciences / The Royal Society》2010,277(1698):3265-3274
Cord-forming fungi form extensive networks that continuously adapt to maintain an efficient transport system. As osmotically driven water uptake is often distal from the tips, and aqueous fluids are incompressible, we propose that growth induces mass flows across the mycelium, whether or not there are intrahyphal concentration gradients. We imaged the temporal evolution of networks formed by Phanerochaete velutina, and at each stage calculated the unique set of currents that account for the observed changes in cord volume, while minimizing the work required to overcome viscous drag. Predicted speeds were in reasonable agreement with experimental data, and the pressure gradients needed to produce these flows are small. Furthermore, cords that were predicted to carry fast-moving or large currents were significantly more likely to increase in size than cords with slow-moving or small currents. The incompressibility of the fluids within fungi means there is a rapid global response to local fluid movements. Hence velocity of fluid flow is a local signal that conveys quasi-global information about the role of a cord within the mycelium. We suggest that fluid incompressibility and the coupling of growth and mass flow are critical physical features that enable the development of efficient, adaptive biological transport networks. 相似文献
39.
Transcriptional regulatory network triggered by oxidative signals configures the early response mechanisms of japonica rice to chilling stress 总被引:3,自引:0,他引:3
40.
Recent studies have revealed an unexpected synergism between two seemingly unrelated protein families: CCN matricellular proteins
and the tumor necrosis factor (TNF) family of cytokines. CCN proteins are dynamically expressed at sites of injury repair
and inflammation, where TNF cytokines are also expressed. Although TNFα is an apoptotic inducer in some cancer cells, it activates
NFκB to promote survival and proliferation in normal cells, and its cytotoxicity requires inhibition of de novo protein synthesis
or NFκB signaling. The presence of CCN1, CCN2, or CCN3 overrides this requirement and unmasks the apoptotic potential of TNFα,
thus converting TNFα from a proliferation-promoting protein into an apoptotic inducer. These CCN proteins also enhance the
cytotoxicity of other TNF cytokines, including LTα, FasL, and TRAIL. Mechanistically, CCNs function through integrin α6β1 and the heparan sulfate proteoglycan (HSPG) syndecan-4 to induce reactive oxygen species (ROS) accumulation, which is essential
for apoptotic synergism. Mutant CCN1 proteins defective for binding α6β1-HSPGs are unable to induce ROS or apoptotic synergism with TNF cytokines. Further, knockin mice that express an α6β1-HSPG-binding defective CCN1 are blunted in TNFα- and Fas-mediated apoptosis, indicating that CCN1 is a physiologic regulator
of these processes. These findings implicate CCN proteins as contextual regulators of the inflammatory response by dictating
or enhancing the cytotoxicity of TNFα and related cytokines. 相似文献