全文获取类型
收费全文 | 14821篇 |
免费 | 1105篇 |
国内免费 | 2篇 |
专业分类
15928篇 |
出版年
2023年 | 62篇 |
2022年 | 63篇 |
2021年 | 163篇 |
2020年 | 134篇 |
2019年 | 141篇 |
2018年 | 248篇 |
2017年 | 302篇 |
2016年 | 373篇 |
2015年 | 493篇 |
2014年 | 580篇 |
2013年 | 771篇 |
2012年 | 1108篇 |
2011年 | 1148篇 |
2010年 | 619篇 |
2009年 | 480篇 |
2008年 | 877篇 |
2007年 | 947篇 |
2006年 | 815篇 |
2005年 | 733篇 |
2004年 | 681篇 |
2003年 | 717篇 |
2002年 | 660篇 |
2001年 | 358篇 |
2000年 | 392篇 |
1999年 | 223篇 |
1998年 | 147篇 |
1997年 | 116篇 |
1996年 | 131篇 |
1995年 | 115篇 |
1994年 | 128篇 |
1993年 | 115篇 |
1992年 | 118篇 |
1991年 | 119篇 |
1990年 | 119篇 |
1989年 | 109篇 |
1988年 | 97篇 |
1987年 | 97篇 |
1986年 | 105篇 |
1985年 | 103篇 |
1984年 | 118篇 |
1983年 | 106篇 |
1982年 | 94篇 |
1981年 | 99篇 |
1980年 | 89篇 |
1979年 | 77篇 |
1978年 | 81篇 |
1977年 | 69篇 |
1976年 | 52篇 |
1974年 | 43篇 |
1973年 | 65篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
102.
Erlend B. Nilsen Henrik Brøseth John Odden John D. C. Linnell 《European Journal of Wildlife Research》2012,58(1):325-333
Harvesting large carnivores is often a controversial issue and thus requires a higher precision than other types of recreational harvest. Despite this, management programmes are often initiated based on very limited knowledge about the state of the population and the composition and magnitude of the harvest. Here we analyse patterns of lynx harvest in Norway based on harvest data from 1994 to 2009. We found clear signs of sex-biased harvest rates, with males generally being more at risk. Further, harvest mortality rates increased with age among males but not among females. In general, the degree of quota filling was high, but there was a tendency for decreasing quota filling with increasing quotas. A population reconstruction of the Norwegian lynx population (1994–2004) based on a modified cohort analysis closely tracked the development in the official monitoring data, suggesting that the managers have access to relatively good information about the development of the population, at least at a larger spatial scale. Despite this, the population trajectory showed large temporal variation, and time lags in the management’s decision-making process are a likely cause of the observed patterns in the Norwegian lynx population. 相似文献
103.
Ettema JF Østergaard S Sørensen MK 《Animal : an international journal of animal bioscience》2011,5(12):1887-1897
The objective of this study was to explore the importance of including genetic progress in milk yield when evaluating different reproductive strategies in a dairy herd by simulation modeling. The model used in this study was SimHerd V, a dynamic and mechanistic Monte Carlo simulation model of a dairy herd including young stock. A daily increasing trend describing genetic milk yield potential of the sire population was included in the model. The inaccuracy of assuming that replacement heifers have the same (milk yield) potential as the cows present in the herd was hereby dealt with. Improving estrus detection rate from 0.45 to 0.80 increased gross margin (GM) per cow-year by €20 when genetic progress was not included in the model. When genetic progress was included in the model, then the same improvement in estrus detection decreased the GM per cow-year by €7.4. This reduced effect was explained by a lower replacement rate in consequence of the improved estrus detection and thereby a slower genetic progress in the herd. There was a reduced effect of including genetic progress on GM when surplus heifers were sold selectively based on breeding values. Repeated insemination with sexed semen on the superior half of all heifers reduced GM by €8 per cow-year when genetic progress was not included and increased the GM by €16 per cow-year when genetic progress was included in the model. Including genetic progress reduced the losses caused by lower conception and estrus detection rates and had a minimal effect with regard to postponing first insemination. This study has proven that it is important to include genetic progress in decisions on reproduction strategies in a dairy herd. 相似文献
104.
The long-lived, light-induced radical YD• of the Tyr161 residue in the D2 protein of Photosystem II (PSII) is known to magnetically interact with the CaMn4 cluster, situated ∼ 30 Å away. In this study we report a transient step-change increase in YD• EPR intensity upon the application of a single laser flash to S1 state-synchronised PSII-enriched membranes from spinach. This transient effect was observed at room temperature and high applied microwave power (100 mW) in samples containing PpBQ, as well as those containing DCMU. The subsequent decay lifetimes were found to differ depending on the additive used. We propose that this flash-induced signal increase was caused by enhanced spin relaxation of YD• by the OEC in the S2 state, as a consequence of the single laser flash turnover. The post-flash decay reflected S2 → S1 back-turnover, as confirmed by their correlations with independent measurements of S2 multiline EPR signal and flash-induced variable fluorescence decay kinetics under corresponding experimental conditions. This flash-induced effect opens up the possibility to study the kinetic behaviour of S-state transitions at room temperature using YD• as a probe. 相似文献
105.
106.
Nadja Schultz-Jensen Berith E. Knudsen Zuzana Frkova Jens Aamand Tina Johansen Jette Thykaer Sebastian R. Sørensen 《Applied microbiology and biotechnology》2014,98(5):2335-2344
The Aminobacter sp. strain MSH1 has potential for pesticide bioremediation because it degrades the herbicide metabolite 2,6-dichlorobenzamide (BAM). Production of the BAM-degrading bacterium using aerobic bioreactor fermentation was investigated. A mineral salt medium limited for carbon and with an element composition similar to the strain was generated. The optimal pH and temperature for strain growth were determined using shaker flasks and verified in bioreactors. Glucose, fructose, and glycerol were suitable carbon sources for MSH1 (μ?=?0.1 h?1); slower growth was observed on succinate and acetic acid (μ?=?0.01 h?1). Standard conditions for growth of the MSH1 strain were defined at pH 7 and 25 °C, with glucose as the carbon source. In bioreactors (1 and 5 L), the specific growth rate of MSH1 increased from μ?=?0.1 h?1 on traditional mineral salt medium to μ?=?0.18 h?1 on the optimized mineral salt medium. The biomass yield under standard conditions was 0.47 g dry weight biomass/g glucose consumed. An investigation of the catabolic capacity of MSH1 cells harvested in exponential and stationary growth phases showed a degradation activity per cell of about 3?×?10?9 μg BAM h?1. Thus, fast, efficient, large-scale production of herbicide-degrading Aminobacter was possible, bringing the use of this bacterium in bioaugmentation field remediation closer to reality. 相似文献
107.
Baek KT Vegge CS Skórko-Glonek J Brøndsted L 《Applied and environmental microbiology》2011,77(1):57-66
The microaerophilic bacterium Campylobacter jejuni is the most common cause of bacterial food-borne infections in the developed world. Tolerance to environmental stress relies on proteases and chaperones in the cell envelope, such as HtrA and SurA. HtrA displays both chaperone and protease activities, but little is known about how each of these activities contributes to stress tolerance in bacteria. In vitro experiments showed temperature-dependent protease and chaperone activities of C. jejuni HtrA. A C. jejuni mutant lacking only the protease activity of HtrA was used to show that the HtrA chaperone activity is sufficient for growth at high temperature or under oxidative stress, whereas the HtrA protease activity is essential only under conditions close to the growth limit for C. jejuni. However, the protease activity was required to prevent induction of the cytoplasmic heat shock response even under optimal growth conditions. Interestingly, the requirement of HtrA at high temperatures was found to depend on the oxygen level, and our data suggest that HtrA may protect oxidatively damaged proteins. Finally, protease activity stimulates HtrA production and oligomer formation, suggesting that a regulatory role depends on the protease activity of HtrA. Studying a microaerophilic organism encoding only two known periplasmic chaperones (HtrA and SurA) revealed an efficient HtrA chaperone activity and proposed multiple roles of the protease activity, increasing our understanding of HtrA in bacterial physiology. 相似文献
108.
109.
110.
Heidi G. Møller Andreas P. Rasmussen Hjalte H. Andersen Kasper B. Johnsen Michael Henriksen Meg Duroux 《Molecular neurobiology》2013,47(1):131-144
Glioblastoma multiforme (GBM) is an incurable form of brain cancer with a very poor prognosis. Because of its highly invasive nature, it is impossible to remove all tumor cells during surgical resection, making relapse inevitable. Further research into the regulatory mechanism underpinning GBM pathogenesis is therefore warranted, and over the past decade, there has been an increased focus on the functional role of microRNA (miRNA). This systematic review aims to present a comprehensive overview of all the available literature on the expression profiles and function of miRNA in GBM. Here, we have reviewed 163 papers and identified 253 upregulated, 95 downregulated, and 17 disputed miRNAs with respect to expression levels; 85 % of these miRNAs have not yet been functionally characterized. A focus in this study has been 26 interesting miRNAs involved in the mesenchymal mode of migration and invasion, demonstrating the importance of miRNAs in the context of the cellular niche. Both oncogenic and tumor-suppressive miRNAs were found to affect target genes involved in cell migration, cytoskeletal rearrangement, invasiveness, and angiogenesis. Clearly, the distinct functional properties of these miRNAs need further investigation and might hold a great potential in future molecular therapies targeting GBM. 相似文献