全文获取类型
收费全文 | 56244篇 |
免费 | 4806篇 |
国内免费 | 4288篇 |
出版年
2023年 | 682篇 |
2022年 | 848篇 |
2021年 | 2695篇 |
2020年 | 2074篇 |
2019年 | 2497篇 |
2018年 | 2340篇 |
2017年 | 1804篇 |
2016年 | 2546篇 |
2015年 | 3629篇 |
2014年 | 4380篇 |
2013年 | 4437篇 |
2012年 | 5286篇 |
2011年 | 4762篇 |
2010年 | 2882篇 |
2009年 | 2599篇 |
2008年 | 2939篇 |
2007年 | 2637篇 |
2006年 | 2262篇 |
2005年 | 1889篇 |
2004年 | 1511篇 |
2003年 | 1421篇 |
2002年 | 1073篇 |
2001年 | 907篇 |
2000年 | 886篇 |
1999年 | 799篇 |
1998年 | 488篇 |
1997年 | 446篇 |
1996年 | 462篇 |
1995年 | 423篇 |
1994年 | 413篇 |
1993年 | 326篇 |
1992年 | 446篇 |
1991年 | 324篇 |
1990年 | 284篇 |
1989年 | 260篇 |
1988年 | 210篇 |
1987年 | 194篇 |
1986年 | 176篇 |
1985年 | 154篇 |
1984年 | 115篇 |
1983年 | 122篇 |
1982年 | 81篇 |
1981年 | 45篇 |
1980年 | 52篇 |
1979年 | 63篇 |
1977年 | 35篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 45篇 |
1972年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Disturbance plays an integral part in generating heterogeneity required for ecosystem persistence, but the increased amplitude and duration of disturbances linked to drivers of global change could result in ecosystem shifts or collapse. Biomonitoring over time provides insights into trajectories of ecosystem change. The responses of two instream animal taxa to two contrasting disturbance events, a major flood event and the long-term cumulative effects of land-use changes, were assessed in 1999–2012 by quantifying variation and change in abundance of functional groups based on flow rate sensitivity, water quality and metrics of ecological condition. All metrics recovered to pre-flood conditions within seven months after the flood event. Similarly, cumulative impacts of land use effected significant decreases in some but not all metrics. Indices that did not change, including SASS total score and ASPT, were the result of insufficient consideration of the decrease in the abundance of sensitive taxa specifically, and the abundance of all taxa in general. The decrease in abundance of sensitive taxa could signal imminent collapse in certain metrics. Evidence is also provided for a shift in the structure of fish assemblages linked to the decrease and loss of taxa sensitive to ecosystem degradation caused by the longer-term impacts of land-use change. 相似文献
32.
33.
Du Jingjing Qv Mingxiang Li Ke Yin Xiaoyun Meng Fanxiao Yang Jingchao Ma Chuang 《Limnology》2019,20(2):173-179
Limnology - The impacts of three commonly used benzophenone-type UV filters including benzophenone (BP), 2-hydroxy-4-methoxy-benzophenone (BP3), and 2-hydroxy-4-methoxy-benzophenone-5-sulfonicacid... 相似文献
34.
<正>Structural variations (SVs) are mutations with large-scale changes (generally50 bp) in the genome. SVs are major sources of the genetic diversity of organisms and thus are of high relevance to phenotype variations, gene dosage and evolutionary genetics. Except detecting SVs through comparative genetic analyses, dozens of software had been developed based on the alignment of short-reads to a single linear genome in the past decades (Guan and Sung, 2016). 相似文献
35.
<正>Geminiviruses are a group of circular single-stranded DNA viruses that constitute the largest family of plant viruses.Many diseases resulting from geminivirus infections, such as maize streak disease, cassava mosaic disease, tomato yellow leaf curl disease and cotton leaf curl diseases, cause significant problems in terms of economic losses, posing a serious threat to global crop productivity (Hanley-Bowdoin et al., 2013; Yang et al., 2019). As obligate intracellular 相似文献
36.
37.
Streptococcus pyogenes is commonly found on pharynx, mouth and rarely on skin, lower gastrointestinal tract. It is a potential pathogen causing tonsillitis, pneumonia, endocarditis. The present study was undertaken to study the effects of low shear modeled microgravity on growth, morphology, antibiotic resistance, cross-stress resistance to various stresses and alteration in gene expression of S. pyogenes. The growth analysis performed using UV–Visible spectroscopy indicated decrease in growth of S. pyogenes under low shear modeled microgravity. Morphological analysis by Bio-transmission electron microscopy (TEM), Bio-scanning electron microscopy (SEM) did not reveal much difference between normal and low shear modeled microgravity grown S. pyogenes. The sensitivity of S. pyogenes to antibiotics ampicillin, penicillin, streptomycin, kanamycin, hygromycin, rifampicin indicates that the bacterium is resistant to hygromycin. Further S. pyogenes cultured under low shear modeled microgravity was found to be more sensitive to ampicillin and rifampicin as compared with normal gravity grown S. pyogenes. The bacteria were tested for the acid, osmotic, temperature and oxidative cross stress resistances. The gene expression of S. pyogenes under low shear modeled microgravity analyzed by microarray revealed upregulation of 26 genes and down regulation of 22 genes by a fold change of 1.5. 相似文献
38.
Dispersive liquid–liquid microextraction (DLLME) coupled with ultra-performance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS) was developed for the extraction and determination of 10 β2-agonists in animal urine. Some experimental parameters, such as the type and volume of the extraction solvent, the concentration of the dispersant, the salt concentration, the pH value of the sample solution, the extraction time and the speed of centrifugation, were investigated and optimized. Under the optimized conditions, a good enrichment factors (4.8 to 32.3) were obtained for the extraction. The enrichment factor show that the concentration rate of DLLME is significantly higher than other pretreatment methods, and the detection sensitivity has been greatly improved. The calibration curves were linear, the correlation coefficient ranged from 0.9928 to 0.9999 for the concentration range of 0.05 to 50 ngmL-1 and 0.1 to 50 ngmL-1, and the relative standard deviations (RSDs, n = 15, intra and inter-day precision) at a concentration of 5 ngmL-1 were in the range of 1.8 to 14.6%. The limits of detection (LODs) for the 10 β2-agonists, based on a signal-to-noise ratio (S/N) of 3, were in the range of 0.01 to 0.03 ngmL-1. The proposed method was used to identify β2-agonists in three types of animal urine (swine, cattle, sheep), and the relative recoveries from each matrix were in the range of 89.2 to 106.8%, 90.0 to 109.8% and 89.2 to 107.2%, respectively. 相似文献
39.
Benjamin S. Chambers Yang Li Richard L. Hodinka Scott E. Hensley 《Journal of virology》2014,88(18):10986-10989
Prior to serological testing, influenza viruses are typically propagated in eggs or cell culture. Recent human H3N2 strains bind to cells with low avidity. Here, we isolated nine primary H3N2 viral isolates from respiratory secretions of children. Upon propagation in vitro, five of these isolates acquired hemagglutinin or neuraminidase mutations that increased virus binding to cell surfaces. These mutations can potentially confound serological assays commonly used to identify antigenically novel influenza viruses. 相似文献