全文获取类型
收费全文 | 3017篇 |
免费 | 192篇 |
国内免费 | 1篇 |
出版年
2021年 | 40篇 |
2020年 | 20篇 |
2019年 | 20篇 |
2018年 | 38篇 |
2017年 | 35篇 |
2016年 | 72篇 |
2015年 | 112篇 |
2014年 | 118篇 |
2013年 | 170篇 |
2012年 | 252篇 |
2011年 | 193篇 |
2010年 | 108篇 |
2009年 | 130篇 |
2008年 | 177篇 |
2007年 | 182篇 |
2006年 | 149篇 |
2005年 | 163篇 |
2004年 | 143篇 |
2003年 | 123篇 |
2002年 | 138篇 |
2001年 | 42篇 |
2000年 | 46篇 |
1999年 | 44篇 |
1998年 | 42篇 |
1997年 | 38篇 |
1996年 | 43篇 |
1995年 | 30篇 |
1994年 | 26篇 |
1993年 | 38篇 |
1992年 | 47篇 |
1991年 | 27篇 |
1990年 | 34篇 |
1989年 | 26篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 27篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 14篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 19篇 |
1977年 | 9篇 |
1976年 | 14篇 |
1975年 | 11篇 |
1968年 | 7篇 |
1967年 | 7篇 |
1966年 | 6篇 |
排序方式: 共有3210条查询结果,搜索用时 15 毫秒
81.
Developmental mechanisms not only produce an organismal phenotype, but they also structure the way genetic variation maps to phenotypic variation. Here, we revisit a computational model for the evolution of ontogeny based on cellular automata, in which evolution regularly discovered two alternative mechanisms for achieving a selected phenotype, one showing high modularity, the other showing morphological integration. We measure a primary variational property of the systems, their distribution of fitness effects of mutation. We find that the modular ontogeny shows the evolution of mutational robustness and ontogenic simplification, while the integrated ontogeny does not. We discuss the wider use of this methodology on other computational models of development as well as real organisms. 相似文献
82.
Ingrid M. Lubbers Matty P. Berg Gerlinde B. De Deyn Wim H. van der Putten Jan Willem van Groenigen 《Global Change Biology》2020,26(3):1886-1898
Soil faunal activity can be a major control of greenhouse gas (GHG) emissions from soil. Effects of single faunal species, genera or families have been investigated, but it is unknown how soil fauna diversity may influence emissions of both carbon dioxide (CO2, end product of decomposition of organic matter) and nitrous oxide (N2O, an intermediate product of N transformation processes, in particular denitrification). Here, we studied how CO2 and N2O emissions are affected by species and species mixtures of up to eight species of detritivorous/fungivorous soil fauna from four different taxonomic groups (earthworms, potworms, mites, springtails) using a microcosm set‐up. We found that higher species richness and increased functional dissimilarity of species mixtures led to increased faunal‐induced CO2 emission (up to 10%), but decreased N2O emission (up to 62%). Large ecosystem engineers such as earthworms were key drivers of both CO2 and N2O emissions. Interestingly, increased biodiversity of other soil fauna in the presence of earthworms decreased faunal‐induced N2O emission despite enhanced C cycling. We conclude that higher soil fauna functional diversity enhanced the intensity of belowground processes, leading to more complete litter decomposition and increased CO2 emission, but concurrently also resulting in more complete denitrification and reduced N2O emission. Our results suggest that increased soil fauna species diversity has the potential to mitigate emissions of N2O from soil ecosystems. Given the loss of soil biodiversity in managed soils, our findings call for adoption of management practices that enhance soil biodiversity and stimulate a functionally diverse faunal community to reduce N2O emissions from managed soils. 相似文献
83.
84.
85.
86.
Jacob Schn Angele Breithaupt Dirk Hper Jacqueline King Anne Pohlmann Rokshana Parvin Klaus-Peter Behr Bernd-Andreas Schwarz Martin Beer Jürgen Stech Timm Harder Christian Grund 《PLoS pathogens》2021,17(4)
Repeated outbreaks due to H3N1 low pathogenicity avian influenza viruses (LPAIV) in Belgium were associated with unusually high mortality in chicken in 2019. Those events caused considerable economic losses and prompted restriction measures normally implemented for eradicating high pathogenicity avian influenza viruses (HPAIV). Initial pathology investigations and infection studies suggested this virus to be able to replicate systemically, being very atypical for H3 LPAIV. Here, we investigate the pathogenesis of this H3N1 virus and propose a mechanism explaining its unusual systemic replication capability. By intravenous and intracerebral inoculation in chicken, we demonstrate systemic spread of this virus, extending to the central nervous system. Endoproteolytic viral hemagglutinin (HA) protein activation by either tissue-restricted serine peptidases or ubiquitous subtilisin-like proteases is the functional hallmark distinguishing (H5 or H7) LPAIV from HPAIV. However, luciferase reporter assays show that HA cleavage in case of the H3N1 strain in contrast to the HPAIV is not processed by intracellular proteases. Yet the H3N1 virus replicates efficiently in cell culture without trypsin, unlike LPAIVs. Moreover, this trypsin-independent virus replication is inhibited by 6-aminohexanoic acid, a plasmin inhibitor. Correspondingly, in silico analysis indicates that plasminogen is recruitable by the viral neuraminidase for proteolytic activation due to the loss of a strongly conserved N-glycosylation site at position 130. This mutation was shown responsible for plasminogen recruitment and neurovirulence of the mouse brain-passaged laboratory strain A/WSN/33 (H1N1). In conclusion, our findings provide good evidence in natural chicken strains for N1 neuraminidase-operated recruitment of plasminogen, enabling systemic replication leading to an unusual high pathogenicity phenotype. Such a gain of function in naturally occurring AIVs representing an established human influenza HA-subtype raises concerns over potential zoonotic threats. 相似文献
87.
Hanne Steel Freija Verdoodt Andrea Čerevková Marjolein Couvreur Pamela Fonderie Tom Moens Wim Bert 《Invertebrate Biology》2013,132(2):108-119
Nematodes are omnipresent in composts and are active in virtually all stages of the composting process. Major shifts in species composition, life strategies, and feeding behavior occur during the composting process. Due to the heat peak, nematodes can be virtually absent, but several taxa appear immediately when the temperatures drop. These comprise both taxa present before the heat peak and new taxa. However, it is not known how nematodes populate the compost. In this study, we aimed to assess the survival and colonization capacity of nematodes in compost. Our results showed that composting processes inaccessible to insects or not in contact with soil did not significantly influence nematode succession during composting. However, differences between treatments were found for some specific taxa (i.e., for Acrostichus sp., Neodiplogasteridae sp., Nygolaimoides sp., and Rhabditidae sp. 1), illustrating the importance of insects for the dispersal of nematodes to compost. Experiments in the lab with the blue bottle fly as a possible carrier demonstrated actual transport of nematodes isolated from compost by the fly (i.e., Halicephalobus cfr. gingivalis, Diploscapter coronatus, Diplogasteritus sp., Acrostichus sp., and Mesorhabditis sp.). Juveniles and dauer stages of Aphelenchoides sp., Panagrolaimus sp., and rhabditids survived an experimentally induced temperature peak, while members of Tylenchidae did not. In conclusion, our results indicate that the rapidly changing nematode community in compost is the result of both differential survival and colonization capacities. 相似文献
88.
Wouter de Munter Arjen B Blom Monique M Helsen Birgitte Walgreen Peter M van der Kraan Leo AB Joosten Wim B van den Berg Peter LEM van Lent 《Arthritis research & therapy》2013,15(6):R178
Introduction
Osteoarthritis (OA) is associated with the metabolic syndrome, however the underlying mechanisms remain unclear. We investigated whether low density lipoprotein (LDL) accumulation leads to increased LDL uptake by synovial macrophages and affects synovial activation, cartilage destruction and enthesophyte/osteophyte formation during experimental OA in mice.Methods
LDL receptor deficient (LDLr−/−) mice and wild type (WT) controls received a cholesterol-rich or control diet for 120 days. Experimental OA was induced by intra-articular injection of collagenase twelve weeks after start of the diet. OA knee joints and synovial wash-outs were analyzed for OA-related changes. Murine bone marrow derived macrophages were stimulated with oxidized LDL (oxLDL), whereupon growth factor presence and gene expression were analyzed.Results
A cholesterol-rich diet increased apolipoprotein B (ApoB) accumulation in synovial macrophages. Although increased LDL levels did not enhance thickening of the synovial lining, S100A8 expression within macrophages was increased in WT mice after receiving a cholesterol-rich diet, reflecting an elevated activation status. Both a cholesterol-rich diet and LDLr deficiency had no effect on cartilage damage; in contrast, ectopic bone formation was increased within joint ligaments (fold increase 6.7 and 6.1, respectively). Moreover, increased osteophyte size was found at the margins of the tibial plateau (4.4 fold increase after a cholesterol-rich diet and 5.3 fold increase in LDLr−/− mice). Synovial wash-outs of LDLr−/− mice and supernatants of macrophages stimulated with oxLDL led to increased transforming growth factor-beta (TGF-β) signaling compared to controls.Conclusions
LDL accumulation within synovial lining cells leads to increased activation of synovium and osteophyte formation in experimental OA. OxLDL uptake by macrophages activates growth factors of the TGF-superfamily. 相似文献89.
90.
The finding that a variety of chemicals display oestrogenic activity has resulted in the development of in vitro and in vivo assays to assess oestrogenic activity. One such assay, the yeast oestrogen assay (YES) makes use of recombinant yeast cells that harbour an oestrogen receptor expression cassette and a reporter construct, coding for bgalactosidase. The induction mechanism starts with the binding of oestrogenic compounds to the oestrogen receptor. This complex activates the production of β-galactosidase. The β-galactosidase activity is thus a measure of the oestrogenic activity of chemical compounds. In the YES assay, the β-galactosidase activity may be quantified with the chromogenic substrate chlorophenol red-β-d-galactopyranoside (CPRG). In the present study it is reported that CPRG or its β-galactosidase degradation product chlorophenol red act in the YES as an oestrogenic compound itself. The implications of this finding are described. It is especially argued that chlorophenol red production after prolonged incubation of the assay might be misinterpreted as an oestrogenic effect of the test compound. 相似文献