全文获取类型
收费全文 | 1082篇 |
免费 | 117篇 |
出版年
2023年 | 12篇 |
2022年 | 7篇 |
2021年 | 60篇 |
2020年 | 24篇 |
2019年 | 28篇 |
2018年 | 38篇 |
2017年 | 35篇 |
2016年 | 54篇 |
2015年 | 99篇 |
2014年 | 84篇 |
2013年 | 99篇 |
2012年 | 119篇 |
2011年 | 109篇 |
2010年 | 46篇 |
2009年 | 37篇 |
2008年 | 42篇 |
2007年 | 56篇 |
2006年 | 43篇 |
2005年 | 29篇 |
2004年 | 29篇 |
2003年 | 27篇 |
2002年 | 22篇 |
2001年 | 4篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1972年 | 3篇 |
1969年 | 2篇 |
1967年 | 2篇 |
1931年 | 2篇 |
排序方式: 共有1199条查询结果,搜索用时 15 毫秒
1.
2.
3.
Sarina Thiel Franziska Willems Nina Farwig Finn Rehling Dana G. Schabo Matthias Schleuning Ney Shahuano Tello Till Töpfer Marco Tschapka Eckhard W. Heymann Katrin Heer 《Biotropica》2023,55(3):650-664
Vertical stratification is a key feature of tropical forests and structures plant–frugivore interactions. However, it is unclear whether vertical differences in plant-frugivore interactions are due to differences among strata in plant community composition or inherent preferences of frugivores for specific strata. To test this, we observed fruit removal of a diverse frugivore community on the liana Marcgravia longifolia in a Peruvian rain forest. Unlike most other plants, Marcgravia longifolia produces fruits across forest strata. This enabled us to study effects of vertical stratification on fruit removal without confounding effects of plant species and stratum. We found a high number of visits of a few frugivore species in the understorey and a low number of visits of many different frugivores in the canopy and midstorey. Whereas partial and opportunistic frugivores foraged across strata with differing frequencies, obligate frugivores were only found eating fruits in the higher strata. Avian frugivores foraging in the canopy were mainly large species with pointed wings, whereas under- and midstorey avian foragers were smaller with rounded wings. Our findings suggest a continuous shift in the frugivore community composition along the vertical gradient, from a few generalized frugivores in the understorey to a diverse set of specialized frugivores in the canopy. This shift in the frugivore community leads to correlated, reciprocal changes from specialized to generalized plant-frugivore interactions. Thus, we conclude that vertical niche differentiation between species in tropical forests persists even when food resources are available across strata. This highlights its role for promoting biodiversity and ecosystem functioning. 相似文献
4.
Temperature Compensation of [U-14C]Glucose Incorporation by Microbial Communities in a River with a Fluctuating Thermal Regime 下载免费PDF全文
Karl-Paul Witzel 《Applied microbiology》1980,39(4):790-796
In summer, the river Saar in the southwest of Germany exhibits distinct temperature fluctuations from 8°C at the source to nearly 30°C in the middle region. Temperature optima for bacterial plate counts and the uptake velocity of [U-14C]glucose by the natural microbial communities of different regions ranged from 20 to 30°C, which is significantly above the mean annual water temperature. A correlation between temperature optima and different seasons or habitats was not observed. Despite the relatively high temperature optima, the turnover time for glucose was shortest at temperatures around the mean annual water temperature, due to changes in the substrate affinity. At limiting substrate concentrations, the higher substrate affinity at lower temperatures may lead to a higher real activity at in situ temperatures, and a compensatory stabilization of uptake rates at fluctuating temperatures is possible. 相似文献
5.
Flemming Kristensen Christoph Walker Florence Bettens Franziska Joncourt Alain L. de Weck 《Cellular immunology》1982,74(1):140-149
When mouse thymocytes are stimulated with PHA, the proliferative response is very low, unless the culture medium is enriched with interleukin 1 (IL-1)- or interleukin 2 (IL-2)-containing supernatants. Cytofluorometric analyses show, however, that PHA stimulation generates a significant number of cells with increased RNA content (transition from the G0 to G1 phase of the cell cycle). If IL-2 is added to such cultures, the activated cells complete their process of RNA synthesis and then enter the S phase. The use of IL-2-containing culture medium thus permits one to obtain a high correlation between the number of g1 cells and [3H]thymidine incorporation (r = 0.97). Enrichment with IL-1-containing supernatants also results in a statistically significant correlation (r = 0.68), but the regression lines are markedly different for the two interleukins (s = 20.3 for IL-2 and s = 9.2 for IL-1), when analyzed after 48 hr of incubation. These observations suggest that the G1 phase must be divided into two subcompartments, G1a and G1b, the G1a-G1b transition being an IL-2-dependent event. If the number of G1b cells is used to establish correlations with [3H]thymidine incorporation, all values fall on the same regression line, regardless of culture conditions and of the addition of interleukins. It is concluded that IL-2 regulates lymphocyte proliferation at the level of RNA synthesis (G1a-G1b transition) rather than that of DNA synthesis (G1-S transition). 相似文献
6.
7.
8.
Song Huijia Jespersen Emil Guo Xiao Du Ning Xie Liujuan Pei Lixin Ye Siyuan Wang Renqing Brix Hans Eller Franziska Guo Weihua 《Hydrobiologia》2021,848(14):3353-3369
Hydrobiologia - Soil salinity diminishes the dominance of species and affects their distribution. Phragmites australis is a dominant ecosystem engineer with broad distribution, high intraspecific... 相似文献
9.
Xie Liujuan Jespersen Emil Ye Siyuan Pei Lixin Song Huijia Guo Xiao Guo Weihua Brix Hans Eller Franziska 《Hydrobiologia》2021,848(14):3331-3351
Hydrobiologia - Phragmites australis is a cosmopolitan plant species with high intraspecific diversity and phenotypic plasticity. Due to its variability and large ecological niche breadth,... 相似文献
10.
Mohammed R. Abdullah María T. Batuecas Franziska Jennert Franziska Voß Philipp Westhoff Thomas P. Kohler Rafael Molina Stephanie Hirschmann Michael Lalk Juan A. Hermoso Sven Hammerschmidt 《Journal of molecular biology》2021,433(2):166723
Nucleotides are important for RNA and DNA synthesis and, despite a de novo synthesis by bacteria, uptake systems are crucial. Streptococcus pneumoniae, a facultative human pathogen, produces a surface-exposed nucleoside-binding protein, PnrA, as part of an ABC transporter system. Here we demonstrate the binding affinity of PnrA to nucleosides adenosine, guanosine, cytidine, thymidine and uridine by microscale thermophoresis and indicate the consumption of adenosine and guanosine by 1H NMR spectroscopy. In a series of five crystal structures we revealed the PnrA structure and provide insights into how PnrA can bind purine and pyrimidine ribonucleosides but with preference for purine ribonucleosides. Crystal structures of PnrA:nucleoside complexes unveil a clear pattern of interactions in which both the N- and C- domains of PnrA contribute. The ribose moiety is strongly recognized through a conserved network of H-bond interactions, while plasticity in loop 27–36 is essential to bind purine- or pyrimidine-based nucleosides.Further, we deciphered the role of PnrA in pneumococcal fitness in infection experiments. Phagocytosis experiments did not show a clear difference in phagocytosis between PnrA-deficient and wild-type pneumococci. In the acute pneumonia infection model the deficiency of PnrA attenuated moderately virulence of the mutant, which is indicated by a delay in the development of severe lung infections. Importantly, we confirmed the loss of fitness in co-infections, where the wild-type out-competed the pnrA-mutant. In conclusion, we present the PnrA structure in complex with individual nucleosides and show that the consumption of adenosine and guanosine under infection conditions is required for virulence. 相似文献