全文获取类型
收费全文 | 4071篇 |
免费 | 458篇 |
国内免费 | 2篇 |
出版年
2021年 | 70篇 |
2020年 | 38篇 |
2019年 | 51篇 |
2018年 | 63篇 |
2017年 | 56篇 |
2016年 | 87篇 |
2015年 | 144篇 |
2014年 | 152篇 |
2013年 | 228篇 |
2012年 | 223篇 |
2011年 | 220篇 |
2010年 | 123篇 |
2009年 | 126篇 |
2008年 | 179篇 |
2007年 | 153篇 |
2006年 | 144篇 |
2005年 | 179篇 |
2004年 | 172篇 |
2003年 | 142篇 |
2002年 | 125篇 |
2001年 | 83篇 |
2000年 | 99篇 |
1999年 | 101篇 |
1998年 | 65篇 |
1997年 | 57篇 |
1996年 | 49篇 |
1995年 | 45篇 |
1994年 | 49篇 |
1993年 | 39篇 |
1992年 | 79篇 |
1991年 | 70篇 |
1990年 | 77篇 |
1989年 | 66篇 |
1988年 | 55篇 |
1987年 | 65篇 |
1986年 | 42篇 |
1985年 | 48篇 |
1984年 | 34篇 |
1983年 | 32篇 |
1982年 | 31篇 |
1980年 | 33篇 |
1979年 | 45篇 |
1978年 | 43篇 |
1977年 | 37篇 |
1975年 | 25篇 |
1974年 | 31篇 |
1973年 | 26篇 |
1971年 | 28篇 |
1969年 | 23篇 |
1967年 | 31篇 |
排序方式: 共有4531条查询结果,搜索用时 176 毫秒
1.
Ecology of Plant‐Derived Smoke: Its Use in Seed Germination by Lara V Jefferson,Marcello Pennacchio and Kayri Havens with illustrations by David S. Sollenberger. Oxford and New York: Oxford University Press, 2014. xi + 316 pp, 18 b&w line diagrams. Hardback with dust jacket. ISBN 978‐0‐19‐975593‐6. £40. 下载免费PDF全文
2.
D. R. Dean R. A. Setterquist K. E. Brigle D. J. Scott N. F. Laird W. E. Newton 《Molecular microbiology》1990,4(9):1505-1512
Metallocluster extrusion requirements, interspecies MoFe-protein primary sequence comparisons and comparison of the primary sequences of the MoFe-protein subunits with each other have been used to assign potential P-cluster (Fe-S cluster) domains within the MoFe protein. In each alpha-beta unit of the MoFe protein, alpha-subunit domains, which include potential Fe-S cluster ligands Cys-62, His-83, Cys-88 and Cys-154, and beta-subunit domains, which include potential Fe-S cluster ligands Cys-70, His-90, Cys-95 and Cys-153, are proposed to comprise nearly equivalent P-cluster environments located adjacent to each other in the native protein. As an approach to test this model and to probe the functional properties of the P clusters, amino acid residue substitutions were placed at the alpha-subunit Cys-62, His-83, Cys-88 and Cys-154 positions by site-directed mutagenesis of the Azotobacter vinelandii nifD gene. The diazotrophic growth rates, MoFe-protein acetylene-reduction activities, and whole-cell S = 3/2 electron paramagnetic resonance spectra of these mutants were examined. Results of these experiments show that MoFe-protein alpha-subunit residues, Cys-62 and Cys-154, are probably essential for MoFe-protein activity but that His-83 and Cys-88 residues are not. These results indicate either that His-83 and Cys-88 do not provide essential P-cluster ligands or that a new cluster-ligand arrangement is formed in their absence. 相似文献
3.
4.
Interactions between water availability and elevated atmosphericCO2 concentrations have the potential to be important factorsin determining future forage supply from temperate pastures.Using large turves from an established pasture, the responseof these communities at 350 or 700 l l1 CO2 to a soilmoisture deficit and to recovery from the deficit in comparisonto turves that were well-watered throughout was measured. Priorto this experiment the turves had been exposed to the CO2 treatmentsfor 324 d. Net CO2 exchange continued at elevated CO2 even when the volumetricsoil moisture content was less than 0.10 m3 m3 soil;at the same moisture deficit gas exchange at ambient CO2 waszero. The additional carbon fixed by the elevated CO2 turveswas primarily allocated below-ground as shown by the maintenanceof root length density at the same level as in well-wateredturves. When the dry turves were rewatered there was compensatorygrowth at ambient CO2 so that the above-ground growth rate exceededthat of turves that had not experienced a moisture deficit.At the start of this experiment, the turves that were growingat 700 l I1 CO2 had a greater proportion of legume (principallywhite clover, Trifolium repens L.) in the harvested herbage.There was a trend for the legume content at elevated CO2 tobe reduced under a soil moisture deficit. The results indicate different strategies in response to soilmoisture deficits depending on the CO2 concentration. At ambientCO2, growth stopped, but plants were able to respond stronglyon rewatering; while at elevated CO2 growth continued (particularlybelow-ground), but no additional growth was evident on rewatering.Ecosystem gas exchange measurements taken at the end of theexperiment (after 429 d of exposure to CO2) showed 33% moreCO2 was fixed at elevated CO2 with only a small (12%) and nonsignificantdownward regulation. Key words: Carbon dioxide, climate change, grassland, gas exchange, soil moisture deficit 相似文献
5.
Priya Gogoi Saedeh Sepehri Yi Zhou Michael A. Gorin Carmela Paolillo Ettore Capoluongo Kyle Gleason Austin Payne Brian Boniface Massimo Cristofanilli Todd M. Morgan Paolo Fortina Kenneth J. Pienta Kalyan Handique Yixin Wang 《PloS one》2016,11(1)
Current analysis of circulating tumor cells (CTCs) is hindered by sub-optimal sensitivity and specificity of devices or assays as well as lack of capability of characterization of CTCs with clinical biomarkers. Here, we validate a novel technology to enrich and characterize CTCs from blood samples of patients with metastatic breast, prostate and colorectal cancers using a microfluidic chip which is processed by using an automated staining and scanning system from sample preparation to image processing. The Celsee system allowed for the detection of CTCs with apparent high sensitivity and specificity (94% sensitivity and 100% specificity). Moreover, the system facilitated rapid capture of CTCs from blood samples and also allowed for downstream characterization of the captured cells by immunohistochemistry, DNA and mRNA fluorescence in-situ hybridization (FISH). In a subset of patients with prostate cancer we compared the technology with a FDA-approved CTC device, CellSearch and found a higher degree of sensitivity with the Celsee instrument. In conclusion, the integrated Celsee system represents a promising CTC technology for enumeration and molecular characterization. 相似文献
6.
7.
Microbial interactions are important for ecosystem function, but occur at the microscale and so are difficult to observe. Previous studies in marine systems have shown significant shifts in microbial community abundance and composition over scales of micrometres to centimetres. This study investigates the microscale abundance distributions of virus-like particles (VLPs) and prokaryotes in the lower reaches of a river to determine the extent to which microscale microbial patchiness exists in freshwater systems. Here we report local hotspots surrounded by gradients that reach a maximum 80 and 107 fold change in abundance over 0.9 cm for prokaryotic and VLP subpopulations. Changes in prokaryotic and VLP hotspots were tightly coupled. There were no gradients at tens of centimetres across the boundary layers, which is consistent with strong mixing and turbulence-driven aggregation found in river systems. Quantification of the patchiness shows a marked asymmetry with patches 10 times greater than background common, but depletions being rare or absent in most samples. This consistent asymmetry suggests that coldspots depleted by grazing and lysis are rapidly mixed to background concentrations, while the prevalence of hotspots indicates persistence against disruption. The hotspot to coldspot relative abundance may be useful for understanding microbial river dynamics. The patchiness indicates that the mean- field approach of bulk phase sampling misses the microbially relevant community variation and may underestimate the concentrations of these important microbial groups. 相似文献
8.
A new method for selecting RNA polymerase mutants 总被引:12,自引:0,他引:12
9.
10.