全文获取类型
收费全文 | 13523篇 |
免费 | 1302篇 |
国内免费 | 3篇 |
出版年
2023年 | 64篇 |
2022年 | 49篇 |
2021年 | 183篇 |
2020年 | 162篇 |
2019年 | 206篇 |
2018年 | 229篇 |
2017年 | 265篇 |
2016年 | 402篇 |
2015年 | 621篇 |
2014年 | 717篇 |
2013年 | 853篇 |
2012年 | 981篇 |
2011年 | 897篇 |
2010年 | 597篇 |
2009年 | 531篇 |
2008年 | 757篇 |
2007年 | 753篇 |
2006年 | 695篇 |
2005年 | 658篇 |
2004年 | 670篇 |
2003年 | 612篇 |
2002年 | 585篇 |
2001年 | 270篇 |
2000年 | 215篇 |
1999年 | 237篇 |
1998年 | 184篇 |
1997年 | 148篇 |
1996年 | 131篇 |
1995年 | 113篇 |
1994年 | 120篇 |
1993年 | 114篇 |
1992年 | 145篇 |
1991年 | 120篇 |
1990年 | 107篇 |
1989年 | 108篇 |
1988年 | 91篇 |
1987年 | 70篇 |
1986年 | 74篇 |
1985年 | 103篇 |
1984年 | 89篇 |
1983年 | 71篇 |
1982年 | 75篇 |
1981年 | 60篇 |
1980年 | 52篇 |
1979年 | 63篇 |
1978年 | 53篇 |
1977年 | 49篇 |
1976年 | 54篇 |
1974年 | 48篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Thomas Arn Hansen Helena Fridholm Tobias Guldberg Fr?slev Kristín Rós Kjartansdóttir Eske Willerslev Lars Peter Nielsen Anders Johannes Hansen 《PloS one》2015,10(11)
Rattus norvegicus (R. norvegicus) are ubiquitous and their presence has several effects on the human populations in our urban areas on a global scale. Both historically and presently, this close interaction has facilitated the dissemination of many pathogens to humans, making screening for potentially zoonotic and emerging viruses in rats highly relevant. We have investigated faecal samples from R. norvegicus collected from urban areas using a protocol based on metagenomic enrichment of circular DNA genomes and subsequent sequencing. We found a new type of papillomavirus, with a L1 region 82% identical to that of the known R. norvegicus Papillomavirus 2. Additionally, we found 20 different circular replication associated protein (Rep)-encoding single stranded DNA (CRESS-DNA) virus-like genomes, one of which has homology to the replication-associated gene of Beak and feather disease virus. Papillomaviruses are a group of viruses known for their carcinogenic potential, and although they are known to infect several different vertebrates, they are mainly studied and characterised in humans. CRESS-DNA viruses are found in many different environments and tissue types. Both papillomaviruses and CRESS-DNA viruses are known to have pathogenic potential and screening for novel and known viruses in R. norvegicus could help identify viruses with pathogenic potential. 相似文献
2.
Cameron L. Aldridge Scott E. Nielsen Hawthorne L. Beyer Mark S. Boyce John W. Connelly Steven T. Knick Michael A. Schroeder 《Diversity & distributions》2008,14(6):983-994
Aim Greater sage‐grouse (Centrocercus urophasianus), a shrub‐steppe obligate species of western North America, currently occupies only half its historical range. Here we examine how broad‐scale, long‐term trends in landscape condition have affected range contraction. Location Sagebrush biome of the western USA. Methods Logistic regression was used to assess persistence and extirpation of greater sage‐grouse range based on landscape conditions measured by human population (density and population change), vegetation (percentage of sagebrush habitat), roads (density of and distance to roads), agriculture (cropland, farmland and cattle density), climate (number of severe and extreme droughts) and range periphery. Model predictions were used to identify areas where future extirpations can be expected, while also explaining possible causes of past extirpations. Results Greater sage‐grouse persistence and extirpation were significantly related to sagebrush habitat, cultivated cropland, human population density in 1950, prevalence of severe droughts and historical range periphery. Extirpation of sage‐grouse was most likely in areas having at least four persons per square kilometre in 1950, 25% cultivated cropland in 2002 or the presence of three or more severe droughts per decade. In contrast, persistence of sage‐grouse was expected when at least 30 km from historical range edge and in habitats containing at least 25% sagebrush cover within 30 km. Extirpation was most often explained (35%) by the combined effects of peripherality (within 30 km of range edge) and lack of sagebrush cover (less than 25% within 30 km). Based on patterns of prior extirpation and model predictions, we predict that 29% of remaining range may be at risk. Main Conclusions Spatial patterns in greater sage‐grouse range contraction can be explained by widely available landscape variables that describe patterns of remaining sagebrush habitat and loss due to cultivation, climatic trends, human population growth and peripherality of populations. However, future range loss may relate less to historical mechanisms and more to recent changes in land use and habitat condition, including energy developments and invasions by non‐native species such as cheatgrass (Bromus tectorum) and West Nile virus. In conjunction with local measures of population performance, landscape‐scale predictions of future range loss may be useful for prioritizing management and protection. Our results suggest that initial conservation efforts should focus on maintaining large expanses of sagebrush habitat, enhancing quality of existing habitats, and increasing habitat connectivity. 相似文献
3.
4.
5.
6.
7.
Lauren Porter Jennifer Duke Meredith Hennon David Dekevich Erik Crankshaw Ghada Homsi Matthew Farrelly 《PloS one》2015,10(5)
Recent youth trends in the prevalence of e-cigarette and traditional cigarette use in Florida were examined in a cross-sectional, representative state sample from 2011 to 2014. Traditional cigarette use among youth declined during the study period. Experimentation with and past 30-day use of e-cigarettes among Florida youth tripled over 4 years. Past 30-day e-cigarette use exceeded traditional cigarette use in 2014; 10.8% of high school and 4.0% of middle school students reported recent e-cigarette use, compared with 8.7% of high school and 2.9% of middle school students for traditional cigarettes (P<0.001). By 2014, 20.5% of high school and 8.5% of middle school students reported ever use of e-cigarettes. Among ever e-cigarette users in 2014, 30.3% of high school and 42.2% of middle school students had never smoked traditional cigarettes. Given the concern that significant rates of e-cigarette use by U.S. adolescents may have a negative effect on public health, further review of e-cigarette advertising, marketing, sales, and use among U.S. youth is warranted. 相似文献
8.
People are biased partisans: they tend to agree with policies from political parties they identify with, independent of policy content. Here, we investigate how physiological reactions to political parties shape bias. Using changes in galvanic skin conductance responses to the visual presentation of party logos, we obtained an implicit and physiological measure of the affective arousal associated with political parties. Subsequently, we exposed subjects to classical party cue experiments where the party sponsors of specific policies were experimentally varied. We found that partisan bias only obtains among those exhibiting a strong physiological reaction to the party source; being a self-reported party identifier is not sufficient on its own. This suggests that partisan bias is rooted in implicit, affective reactions. 相似文献
9.
10.
Ami Klein Sharon Ramcharitar Nevena Christeff Erik Nisbett-Brown Emmanuel Nunez Aaron Malkin 《In vitro cellular & developmental biology. Animal》1991,27(4):307-311
Summary These authors attempted to test the effect of anticoagulants on lymphocytes viability by reproducing the procedure used for
lymphocyte isolation for various immunologic tests in which blood specimens are allowed to stay at room temperature for 2
h before lymphocytes are isolated. Blood was obtained with three different anticoagulants i.e. heparin, citrate, and CPDA
(citrate, phosphate, dextrose, and adenine). Plasma was lyophilized and extracted with ethanol. Dried ethanol extracts were
suspended in medium (RPMI 1640+10% fetal bovine serum) and incubated with a lymphocyte cell line (MOLT-4). After 24 h of incubation
the viability of cells was examined. The following death rates of the cells were observed: heparin −63±4.6% (mean±SEM), citrate
−27±6.7%, and CPDA 6.2±0.6% (P<0.0005). A significant correlation was found between these results and changes in the concentrations of free fatty acids
in the extracts. These results emphasize the importance of choosing the right anticoagulant when the viability of lymphocytes
is obligatory. 相似文献