全文获取类型
收费全文 | 18549篇 |
免费 | 1523篇 |
国内免费 | 8篇 |
专业分类
20080篇 |
出版年
2024年 | 22篇 |
2023年 | 141篇 |
2022年 | 307篇 |
2021年 | 549篇 |
2020年 | 319篇 |
2019年 | 414篇 |
2018年 | 506篇 |
2017年 | 385篇 |
2016年 | 704篇 |
2015年 | 1153篇 |
2014年 | 1244篇 |
2013年 | 1443篇 |
2012年 | 1730篇 |
2011年 | 1516篇 |
2010年 | 980篇 |
2009年 | 818篇 |
2008年 | 1075篇 |
2007年 | 1047篇 |
2006年 | 936篇 |
2005年 | 859篇 |
2004年 | 808篇 |
2003年 | 651篇 |
2002年 | 649篇 |
2001年 | 134篇 |
2000年 | 109篇 |
1999年 | 141篇 |
1998年 | 133篇 |
1997年 | 104篇 |
1996年 | 95篇 |
1995年 | 75篇 |
1994年 | 71篇 |
1993年 | 57篇 |
1992年 | 74篇 |
1991年 | 52篇 |
1990年 | 54篇 |
1989年 | 74篇 |
1988年 | 47篇 |
1987年 | 40篇 |
1986年 | 35篇 |
1985年 | 46篇 |
1984年 | 40篇 |
1983年 | 34篇 |
1981年 | 21篇 |
1980年 | 25篇 |
1979年 | 27篇 |
1976年 | 24篇 |
1975年 | 21篇 |
1974年 | 26篇 |
1973年 | 27篇 |
1968年 | 47篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Stefano Chiari Marco Bardiani Agnese Zauli Sönke Hardersen Franco Mason Laura Spada Alessandro Campanaro 《Journal of Insect Conservation》2013,17(6):1255-1265
Quantitative information on population size, structure and dynamics are urgently needed for assessing species extinction risk and developing monitoring measures, especially for beetles belonging to the threatened guild of saproxylic organisms. Here freshly cut log piles (FCLP) were tested for the monitoring of the longhorn beetle Morimus asper (Sulzer, 1776) (Coleoptera: Cerambycidae). Novel mathematical models based on presence–absence or count data were compared with the most commonly used capture–recapture methods to investigate if less invasive and easier approaches may be suitable for large-scale monitoring of this species and other large saproxylic beetles. The use of FCLP as bait was reliable to detect the presence and abundance of M. asper, and capture events were not influenced by the phenomena of trap-happiness or trap-shyness. In order to obtain accurate presence–absence estimates of M. asper at a reasonable cost piles of at least 0.25 m3 are suggested. Models which take into account the abundance of the species without marking individuals were the best compromise between costs and accuracy of estimation. Therefore, a reliable assessment of the population size of M. asper, can be based on count data without marking individuals. Because FCLP attract adults of M. asper and other saproxylic beetles, they could act also as potential ecological traps if chipped and/or removed after the sampling season. Therefore, if FCLP are used in monitoring as a part of conservation programmes, the piles should be left until completely decayed. 相似文献
172.
We found a new non-native haplotype of Phragmites australis in North America that provides convincing evidence for multiple introductions of this highly invasive reed from Europe. Prior to our detection of this new non-native haplotype, invasion of North America by this reed grass was thought to be limited to a single cp-DNA haplotype–haplotype M. However, we found two sites colonized by haplotype L1 in Quebec, Canada, a haplotype native to northern Europe, Great Britain and Romania. Because the invasion of North America by P. australis is ongoing, and because there is evidence for intra- and inter-specific hybridization and increased fecundity resulting from outcrossing, more attention should be paid to genetic differences and associated vigor of populations of introduced Phragmites across North America. 相似文献
173.
174.
Cecilia Marini Barbara Salani Michela Massollo Adriana Amaro Alessia Isabella Esposito Anna Maria Orengo Selene Capitanio Laura Emionite Mattia Riondato Gianluca Bottoni Cinzia Massara Simona Boccardo Marina Fabbi Cristina Campi Silvia Ravera Giovanna Angelini Silvia Morbelli Michele Cilli Renzo Cordera Mauro Truini Davide Maggi Ulrich Pfeffer Gianmario Sambuceti 《Cell cycle (Georgetown, Tex.)》2013,12(22):3490-3499
Emerging evidence suggests that metformin, a widely used anti-diabetic drug, may be useful in the prevention and treatment of different cancers. In the present study, we demonstrate that metformin directly inhibits the enzymatic function of hexokinase (HK) I and II in a cell line of triple-negative breast cancer (MDA-MB-231). The inhibition is selective for these isoforms, as documented by experiments with purified HK I and II as well as with cell lysates. Measurements of 18F-fluoro-deoxyglycose uptake document that it is dose- and time-dependent and powerful enough to virtually abolish glucose consumption despite unchanged availability of membrane glucose transporters. The profound energetic imbalance activates phosphorylation and is subsequently followed by cell death. More importantly, the “in vivo” relevance of this effect is confirmed by studies of orthotopic xenografts of MDA-MB-231 cells in athymic (nu/nu) mice. Administration of high drug doses after tumor development caused an evident tumor necrosis in a time as short as 48 h. On the other hand, 1 mo metformin treatment markedly reduced cancer glucose consumption and growth. Taken together, our results strongly suggest that HK inhibition contributes to metformin therapeutic and preventive potential in breast cancer. 相似文献
175.
Kurt Reifenberg Fei Cheng Laura Twardowski Ines Küpper Elena Wiese Franziska Bollmann Hartmut Kleinert Manfred Blessing Karl J. Lackner Michael Torzewski 《PloS one》2013,8(12)
Clinical data have indicated a negative correlation between plasma TGFß1 concentrations and the extent of atherosclerosis and have thus led to the hypothesis that the pleiotropic cytokine may have anti-atherogenic properties. T-cells are currently discussed to significantly participate in atherogenesis, but the precise role of adaptive immunity in atherogenesis remains to be elucidated. TGFß1 is known to strongly modulate the function of T-cells, however, inhibition of TGFß1 signalling in T-cells of atherosclerosis-prone knock-out mice failed to unequivocally clarify the role of the cytokine for the development of atherosclerosis. In the present study, we thus tried to specify the role of TGFß1 in atherogenesis by using the murine CD2-TGFß1 transgenic strain which represents a well characterized model of T-cell specific TGFß1 overexpression. The CD2-TGFß1 transgenic mice were crossed to ApoE knock-out mice and quantity and quality of atherosclerosis regarding number of macrophages, smooth muscle cells, CD3 positive T-cells and collagen was analyzed in CD2-TGFß1 ApoE double mutants as well as non-transgenic ApoE controls on both normal and atherogenic diet of a duration of 8, 16 or 24 weeks, respectively. In all experimental groups investigated, we failed to detect any influence of TGFß1 overexpression on disease. Total number of CD3-positive T-lymphocytes was not significantly different in atherosclerotic lesions of CD2-TGFß1 ApoE−/− females and isogenic ApoE−/− controls, even after 24 weeks on the atherogenic diet. The synopsis of these data and our previous study on TGFß1 overexpressing macrophages suggests that potential effects of TGFß1 on atherosclerosis are most probably mediated by macrophages rather than T-cells. 相似文献
176.
177.
Pablo Delgado-Sánchez Laura Yá?ez-Espinosa Juan Francisco Jiménez-Bremont Leonardo Chapa-Vargas Joel Flores 《PloS one》2013,8(11)
Background
Cacti establish mostly occurs under the canopy of nurse plants which provide a less stressful micro-environment, although mechanisms underlying this process are unknown. The impact of the combination of light and watering treatments on Opuntia streptacantha (Cactaceae) seedlings was examined.Methods/Principal Findings
Ecophysiological [titratable acidity, osmotic potential (‘solute potential’, Ψs), relative growth rate (RGR) and their components (NAR, SLA, and LWR)], anatomical (chloroplast density, chloroplast frequency, and cell area), and environmental [photosynthetic photon flux density (PPFD) and air temperature] sets of variables were analyzed, assessing relationships between them and measuring the intensity of the relationships. Three harvests were carried out at days 15, 30, and 45. Ψs and acidity content were the most important responses for seedling establishment. The main anatomical and environmental variables were chloroplast density and water availability, respectively. Opuntia streptacantha seedlings establish better in the shade-watering treatment, due to higher Ψs and acidity, unaffected chloroplasts, and lower PPFD. In addition, the chloroplasts of cells under high-light and non-watering treatment were clumped closer to the center of the cytosol than those under shade-drought, to avoid photoinhibition and/or to better distribute or utilize the penetrating light in the green plant tissue.Conclusions
Opuntia seedlings grow better under the shade, although they can tolerate drought in open spaces by increasing and moving chloroplasts and avoiding drastic decreases in their Ψs. This tolerance could have important implications for predicting the impact of climate change on natural desert regeneration, as well as for planning reforestation-afforestation practices, and rural land uses. 相似文献178.
Laura Solforosi Michela Milani Nicasio Mancini Massimo Clementi Roberto Burioni 《朊病毒》2013,7(2):99-108
Prions are infectious proteins that are responsible for transmissible spongiform encephalopathies (TSEs) and consist primarily of scrapie prion protein (PrPSc), a pathogenic isoform of the host-encoded cellular prion protein (PrPC). The absence of nucleic acids as essential components of the infectious prions is the most striking feature associated to these diseases. Additionally, different prion strains have been isolated from animal diseases despite the lack of DNA or RNA molecules. Mounting evidence suggests that prion-strain-specific features segregate with different PrPSc conformational and aggregation states. Strains are of practical relevance in prion diseases as they can drastically differ in many aspects, such as incubation period, PrPSc biochemical profile (e.g., electrophoretic mobility and glycoform ratio) and distribution of brain lesions. Importantly, such different features are maintained after inoculation of a prion strain into genetically identical hosts and are relatively stable across serial passages. This review focuses on the characterization of prion strains and on the wide range of important implications that the study of prion strains involves. 相似文献
179.
Ilhem Messaoudi Jennifer Vomaske Thomas Totonchy Craig N. Kreklywich Kristen Haberthur Laura Springgay James D. Brien Michael S. Diamond Victor R. DeFilippis Daniel N. Streblow 《PLoS neglected tropical diseases》2013,7(7)
Chikungunya virus (CHIKV) is a re-emerging mosquito-borne Alphavirus that causes a clinical disease involving fever, myalgia, nausea and rash. The distinguishing feature of CHIKV infection is the severe debilitating poly-arthralgia that may persist for several months after viral clearance. Since its re-emergence in 2004, CHIKV has spread from the Indian Ocean region to new locations including metropolitan Europe, Japan, and even the United States. The risk of importing CHIKV to new areas of the world is increasing due to high levels of viremia in infected individuals as well as the recent adaptation of the virus to the mosquito species Aedes albopictus. CHIKV re-emergence is also associated with new clinical complications including severe morbidity and, for the first time, mortality. In this study, we characterized disease progression and host immune responses in adult and aged Rhesus macaques infected with either the recent CHIKV outbreak strain La Reunion (LR) or the West African strain 37997. Our results indicate that following intravenous infection and regardless of the virus used, Rhesus macaques become viremic between days 1–5 post infection. While adult animals are able to control viral infection, aged animals show persistent virus in the spleen. Virus-specific T cell responses in the aged animals were reduced compared to adult animals and the B cell responses were also delayed and reduced in aged animals. Interestingly, regardless of age, T cell and antibody responses were more robust in animals infected with LR compared to 37997 CHIKV strain. Taken together these data suggest that the reduced immune responses in the aged animals promotes long-term virus persistence in CHIKV-LR infected Rhesus monkeys. 相似文献
180.
William R. Kirkpatrick Nathan P. Wiederhold Laura K. Najvar Thomas F. Patterson 《Current fungal infection reports》2013,7(1):68-78
Animal models have long been used to explore various pathophysiological, immunological and microbiological questions in the field of medical mycology. These models have been adapted and altered over time, yet their use has persisted. They remain valuable as research tools due to similarities to processes in human physiology and disease, and are evolving to include more fungal pathogens and infections that better mimic disease in humans. Animal availability, animal cost, housing requirements, the need for immunosuppression, the potential for tissue, fluid or blood samples, a researcher’s familiarity with the model, as well as governmental or institutional regulations, must all be considered when selecting an appropriate one to use. Although the questions of interest have changed over the past 30 years, one idea persists: animal models are valuable tools in research that span the gap between the bench and the clinic. 相似文献